Honestly, I find FEH to be pretty well paced in terms of being able to accomplish stuff being F2P. Im sitting on ~80k feathers right now and have already promoted maybe 8 or 9 units from 4->5 for either skill inheritance or to use them. Arena, Arena Assault, daily missions, and monthly events (Tempest Trials and Voting Gauntlet) easily net in the 30-40k feather range per month. You can even cap out rewards on Tempest Trials through auto-ing the middle difficulties and spending a few stamina potions without much effort at all.
As per merging 5* units, keep in mind they are technically "worth" 5 orbs regardless of how you obtained them (summoning or event rewards). And you also have to weigh if that 5* has optimal IV's for their use. My general rule is that making a Unit +1, for example, isn't as important as giving a unit that "free" 5 orb ability they have to make that new unit as functional a unit as the original unit. But that also only works for rarer skills like Swordbreaker, Quick Riposte, Vantage, etc.
You honestly get the most bang in hoarding common 4* units and making them +10 - especially colorless.
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