Should I hm farm before foddering off?

You should always cap a units merit before foddering off the last copy of them.

Farm farm farm. It only takes a couple hours then you're done for good (unless IS finally decides to increase the HM cap).

Imo, Lewyn's too good of a unit to fodder off if you only have one copy of him.

But if you insist on foddering him, grind up those feathers first.

Depends on if you're fine grinding those feathers. I personally don't like grinding for a unit I don't really care about, so I just level them up fully and then send em off to someone else.

It is a solid amount of feathers though if you do grind.

Don’t forget dragonflowers as well. 5000 feathers HM is worth it!

Unless you are truly anxious to transfer the skill, harvest HM and dragon flowers. This will also give you time to decide which unit is the better inheritor of the skill (after seeing it in action), and/or if you want to fodder the unit at all.

This comes from someone who has no qualms in foddering his only copy of any character, and considers most new units as fodder.