Best usage of feathers for fairly new player.

i'd say focus on three units to +10 merge ( units post cyl such as shigure , libra and soleil are ideal because of inflated stats , armors or dragons if you are aiming for hight tier arena ) . If arena IS your goal , you should focus on getting the askr trio to 5* rarity asap . If not , you can clear most of their quests at 4* rarity . If you could share your units it would really help , though

Promote your askr units to 5 stars, that costs 60k feathers and they are always going to in your inventory no matter what.

After that, just save feathers unit you have 220k, trust me itll be hard having that much conviction the first time around, but by the time you get to 220k feathers you should already know what unit you want to make 5+10, and with 220k feathers you can do that.

You can get into and stay in tier 20 of arena with just your favorites provided they have enough merges on them, all being +10, as well as an invested askr unit.

But really, just save your feathers. Just because you CAN use them right away doesn't mean you should.

If you are a new player, what you need id good units. What to do with your feathers ? Keep them. Then when you get a good IV character, promote him/her. Characters such as Reinhardt or Sheena, who are easy to get and are very powerful with minimum investment. A soleil with +spd and a firesweep sword can do wonders. But overalk Reinhardt should be a priority. Do not forget that you get free 5* stars such as Fjorm or Legendary Ike. Start a team with them, maybe a healer, and a unit you lvl up for minimum efforts. If you are starting, ignore expensive builds and do not invest in the Askr trio. Its an arena thing you do not need. You need one or more effective teams. Also, try to get a proper dancer. They are great force multiplier. Olivia is ok to start and easy to get. Later, you will get better ones.

I think that the best use of your feathers would be to promote the Askr trio so that you always have a bonus unit for the Arena. I think that you should be able to reach anywhere between Tier 15-17 (maybe higher) on just the free 5-Star units that the game provides (with L!Ike, Fjrom, a Brave Hero, and Askr tio unit), and you can get a good amount of orbs and feathers every week. The story maps will always stay, and BHB, GHB, (and possibly LHB if the L!Grima maps are any indication) maps will come back in the future; however, you get more rewards the longer you remain competitive in the Arena, so it's worth to at least invest a bit there before anything else. You don't even need premium investments in any of them--just something common in the 4-Star pool to fill up any missing skill slots that can be useful will do.

Once you find yourself unable to climb up any further (and not drop Tiers), then I think its worth investing feathers to promote GHB / common units like Xander, Camus, Arvis, Reinhardt, etc. for in-game content. The highest Tiers of the Arena will require that you have heavily merged Arena cores, and it will be a long while before you amass the 660K feathers to max merge all of them. You could save up and wait it out if you really wanted to, but I think that you should spend at least a some so that you have competent units for every mode outside of the Arena. (If you play well and invest in the proper units, then any in-game content in FEH can be cleared with 5-Star units without merges, or even 4-Star units--merged or un-merged.)

Once you feel that you have enough good units to clear most or all content that the game throws at you, then focus on saving feathers for max-merging your Arena core.

You seem like a pretty smart and conscientious player. I guessed that you have around 30-40 units at 5*, and your comment about being just short of getting through arena assault makes me think I wasn't too far off.

Up to that point I'd say you're definitely still in the opening phase of an account when you can go for pretty much anything, just explore and develop your knowledge of the game and discover your preferred play style. I think you can continue with it, try out characters you might like, start thinking about merge projects but not quite committing yet because 1.5 months in is too early if you ask me. Players who understand the game and understand themselves and how they need to approach the game in that amount of time are, I wager, about as common as child prodigies who make international performing careers as teenagers.

Maybe offer us a few names of the characters you have most enjoyed using, and we can offer you some suggestions for characters to try at +0 that you might merge to +10?

Also, if you give me your friend code I'll offer you a merged unit to use in relay defense.

You could promote Nowi. She pretty much carried me through the game and is still my strongest unit. She can even be summoned at 3* besides 4*, so she is easy to get alot of copies of.
The Askr Trio is also pretty important to promote if you want to get to the higher tiers and you don't want to spend money to get those weeks bonus heroes.

Usually whenever someone starts I always recommend the same couple units to five star: Nino, Nowi, Soleil, and Reinhardt. However, if you think you already know your playstyle you can go different routes, or if you're having issues with a particular unit then you can five star a counter. For instance, I tend to prefer mages, so I five starred Nino and Tharja off the bat, but if you were to prefer a more defensive playstyle, then I'd recommend units like Sheena or a mix tank Nowi. If you want you can work on some gimmicky builds for the hell of it too. Just remember that everything isn't all about arena, and that you want to ultimately have fun building and using your units in the end.