So...

I've never pulled a Reinhardt, and he is a four star unit to.

I'm not quite sure what you're asking? If you want unit recommendations, you've already given yourself some. If you want to know how to know how to have a more diverse selection of units, you've already answered that question for yourself too.

Just focus on the unit(s) once you get them. You can never have too many lvl 40s, after all. Obviously the ones that come in 4* versions will be much easier to get your hands on and with good IVs, while you need to wait for focuses or dumb luck for the 5* exclusive units.

Don't try and push against your playstyle either just because you think you need x or y, unless there's a hole you're trying to fill. Even then, it's important to keep your playstyle in mind. For example, a hyper offense mage will have weird synergy with a mostly defensive melee team. A dragon or a bulky mage who baits might be more appropriate.

Comments on your current interests.

- Bartre: Infantry axe wielders have trouble in the current meta but he’s still a very strong brusier. If you invested in him I think you’d get decent returns on that investment, especially if he were partnered with a competent mage duelist. Actually, his magic problems might be largely mitigated by the new Deflect Magic 3 seal.

- Cordelia: I haven’t made use of her myself but I believe that she’s widely considered the best budget quad-attacker.

- Est: Lots of people have passed her over but she’s something of a sleeper hit. She can be an incredible nuke with the right set-up and that’s even more true now that DEF Ploy is more broadly available.

- Faye: I’ve never pulled her… But she’s definitely got potential.

- Gwendolyn: I think she’s extremely underrated in a support role as the core of a pure armor emblem team. She has fantastic DEF and good RES. I’ll share my Healer Gwendolyn after this post.

- Hana: Lots of people love her… But I don’t like melee glass cannons unless they’re mounted.

- Hector: To be honest, the power creep is starting to get to him. He’s still strong but definitely not the dominant force he once was.

- Innes: Fantastic anti-mage archer!

- Lilina: She’s tied for highest ranged ATK in the game and is a very strong unit. I’ll share the two builds I most often use on her below.

- Lukas: I would definitely recommend using him. He’s a fantastic physical wall.

- Nowi: She’s one of the most powerful units in the game…even now. Dragons have also been spared from the nerfs going around and thus have gotten a stealth boost.

- Oscar: He has a great stat-line for offensive cavalry but needs a ton of skill inheritance to get there.

- Raven: Much like Barte, he suffers as an infantry axe wielder but definitely has potential as the meta shifts.

- Robin (M): One of my first 5* units… I still make regular use of him. He’s still the best blue TA/Raven user.

- Roy (Brave Heroes): There are few units I hate facing on arena defense more than Brave Roy with dancer support. He’s a beast. I’d definitely recommend investing in him.

- Sonya: She’s in an awkward place stat-wise, stuck between Nino and Deirdre/Julia, but her legendary tome makes the difference. She can be a good nuke and mage duelist with the right investment.

- Tana: I haven’t used her but her legendary weapon seems fantastic.

- Tharja: She’s in an awkward place stat-wise. I wouldn’t recommend investing unless you have one that’s +SPD.

Recommendations for diversity.
- Fliers: Prioritize getting a flying mage if you can, it revolutionizes flier emblem. H!Nowi will fit the bill. S!Corrin running a blade tome completely changed flier emblem for me...and that's without having hone fliers.
- Magic Users: Nino is available at 3* and still one of the strongest all purpose nukes in the game. I'd check IVs until you pull one with +SPD/-DEF or +SPD/-HP. Otherwise... Arvis, Cecilia, Ursula, and other GHB picks are generally good.
- Dragon Users: Y!Tiki, Nowi, and Fae are fantastic. A!Tiki has a niche use as a wall. F!Corrin suffers from the fact that debuffing is not a good primary role in this game...it's better as a secondary function.
Green: I'd recommend Hawkeye and Frederick for melee. For ranged, Nino is of course fantastic and widely available. Boey is, in my opinion, the best TA/Raven mage in the game currently. Cecilia is widely available and a good starting investment as you can then inherit her tome to someone else.
Blue: I don't make much use of physical blues outside of Azura, Sharena, and Gwendolyn... I did have fun running Lukas for awhile but then sacrificed him to Gwendolyn. But Nephenee is reported to be excellent. Tailtiu is now available at 4* and is worth investing in, in regards to stats she's somewhere between Nino and Tharja. Linde and Deltha are also excellent. Reinhardt is easy mode (don't fall for the temptation).

from Kaihaku:

"- Oscar: He has a great stat-line for offensive cavalry but needs a ton of skill inheritance to get there."

Agreed. If you're sitting on a 5* Roderick, you can pretty much pass over his Firesweep Lance+, and take Drag Back from Donnel for his B skill. As for A, you can slap Fury on him—the 6-damage ping from combat won't hurt him much if you will just use him to hook opponents closer to people who can kill whatever he fished. Add cavalry bonuses and go to town.

Thoughts on the units you asked about specifically:

Bartre - not worth your trouble. If you want a physical green unit, build 4* merges of Cherche or Frederick (or, obviously, Hector or Amelia). Useful Fury 2 fodder if you're in a Hinata drought.

Cordelia - Great unit and well worth building if you find one with a good IV set. I like fliers the most for brave weapon nukes since Drag Back / Hit & Run will almost always proc for them and their general reposition game is stellar.

Ephraim - Have never had one but highly useful as the babysitter of an infantry blade tome squad. Less relevant due to P!Azura, split rallies and the broadening pool of sacred seals. Basically, nice to have but not a priority.

Est - 4* merge her and you have a poor man's Cordelia.

Faye - Most useful as fodder to give units with better offensive spreads or a debuff build firesweep bow.

Gwendolyn - Needs to be run in a pure emblem team but can be a tremendous physical wall in that composition. A 4* merge candidate.

Hana - Mostly useful as L&D fodder. I would not sink a ton of resources into an infantry sword without native DC. See my earlier comment about movement types best suited for brave weapons.

Hector - Still a monster. One of the three units every arena team needs to have a ready answer for along with B!Lyn and Reinhardt.

Innes - Have not summoned one but suspect he is best used for PvE content unless you lack for bow users.

Lilina - One of several viable candidates for a 4* merge and Raventome build. Sophia is arguably better because she has more def; but Lilina is perfectly usable as well.

Lukas - a strong infantry tank who can be used if pulled but should not be a priority.

Nowi - Absolute monster, worth building and eventually running as a 5* merge.

Oscar - Not worth your trouble.

Raven - See Bartre. Different strategy but still outclassed by a world of Axe users.

Robin (M) - useful on his own with the right spread and also fantastic for SI.

Roy (Brave Heroes) - He's a fun damage carry who can be very useful in PvE content. Can also be useful in arena if you build your team with him in mind. He is not "plug and play" the way that truly elite units are though.

Sonya - Can't have enough Reinhardt counters. Nasty with Moonbow and (if you're feeling like making it rain), Wrath 3.

Tana - Great stats, great legendary weapon. Probably more of a mixed-comp tank than an ideal flier emblem member.

Tharja - Still great with the right IVs, particularly for PvE content. If you get one with bad IVs, H!Nowi and Katarina both very much appreciate her tome.

My general advice based on what you've said would be to always save up to do full summoning rounds and not try to snipe. You have enough needs that most units will be useful. Try to make 4* merges with good budget builds for units that don't need to be 5* to unlock their potential and save your premium skills and feathers for units that give you an appreciable skill or weapon at 5*. Oh, and never send home a 5* unit.