To Sacrifice or to Use

In my case i try to keep at least 1 copy of every character, even 4* since i'll merge them to 4*+10 before starting to send them home, with few exeptions for 4* that have valuable skills. If a unit has his role outclassed by other units that i have already built then i keep him just for AA while waiting for possible weapn refine that can create his own niche.
In my opinion L!Hector and L!Ike should be used as fodder as they have a very strong and valuable A slot, may i can consider merging L!Hector since there is normal Hector in the summoning pool that gives DC, but that's only if you don't have no unit that desperately needs DC

I don't recommend 5* merges for any unit that you don't have a colossal heart for. If they are 3*/4*, it will take months of feathers; if they are 5* exclusive, it will take even more months of orbs.

For 5* exclusives, if you will not merge to +10, then I would fodder unless there's nobody to fodder to (really? those are amazing skills) or the unit doesn't have much fodder value (for me, L.Ephraim's stance skill is not good because atk/def bond is not hard to have active on enemy phase and will sometimes work even on player phase, and it even gives 1 more point to both stats)

Remember, you can always save them for a specific use later, but once gone, they're gone for good.

You could Ally Support W Tharja and Noire for a fun Mother/Daughter shenanigans team XD Same for Corrin & Kana/Robin & Morgan etc.

As for multiples of someone like Hector. I like keeping separate copies with different builds so I don't have to constantly change him around for my Tempest and Arena teams. ^w^ Finally, I also use the multicolored hearts to keep track of which unit is for which team.

I level 1 copy of every 5 star, even if I'm going to fodder them just to unlock their message. If they have good fodder and are 5 star exclusive I save them, unless I really want to use them.

No good fodder, or if it's on the 4 stars version. I merge them.

From your long list I'd merge: Ryoma, Xander, Ephraim, Tana, Azura, S!Chrom, Minerva, B!Cordelia, Kagero, M!Morgan, Linde, Reinhardt, PA!Inigo, Y!Tiki, Fae, Nowi, F!Grima. They all have skills that are available from 4 stars or just aren't extremely useful.

I'd hang onto all !Ike, and Hector S+ Fodder, then both Ninian. S!Tana's Atk/Spd push is extremely niche, Nino is an easy source of bladetome but unlike Nowi she doesn't need merges to shine.

I usually make this decision unit.by unit. Leif is a good example. I got him instead of swordrein and he was -spd +def. Unusual but manageable. I thought how to make his work with Lightbrand and built him with Fury, threat res, seal atk/Def and threat Spd seal. So now he's51 atk, 39 Def and 38 speed. He's kind of a tank and now joins the battle with Micaiah (which I didn't think I would use it either) for a armor killer team.
So it's about liking something in a unit and thinking a way to build it in a team strategy and sinergy.
Martha is another example. I always thought him as a stupid unit. But one day, in a Grand conquest, I saw a Marth with falchion refine and drive atk and Spd. So he was giving +5 atk/Def and +2 other to everybody around him while regenerating and dealing with stray dragons. Wow, that's game So I built one +atk and he now moves around with Leif. So Leif's speed boon is totally reverted.
Strangest example so far is Tana (+atk/-spd). She's pretty useless cause can't stand hits and won't double. So I built a Leon with Oscar's skills (rally Spd/def and spur Spd/Def) so Tana would stand more hits. For the cherry on the cake I gave him Clarissa's Bow for enemies to be -5 Atk/Spd. NownTana survives and perform really well, even being and outclassed unit.