The problem is, Amorim could just be another 2 year project that doesn't work out like all of our post-SAF managers. Rebuilding the squad towards a niche formation that will be redundant if Amorim fails is a big risk really, if it involves getting rid of potentially very good players that would have come in useful for us under any other manager.
If we are letting him go, I would hope it's with the view that we don't actually think he has what it takes to become an elite attacking player in a PL/CL winning side (in any formation). Whether that be not having the skillset or brain to play in a top team that keeps possession of the ball.
I think Garnacho's ceiling is probably a player who is productive in terms of numbers but very messy in terms of general play, passing, touch, decision making - maybe some similarities to Rashford in that regard (back when Rashford was still a footballer). It wouldn't surprise me if he goes on to have a 20+ goal season in all competitions because he is a goalscoring threat even if his finishing can be frustrating right now, but he'll probably never be an elite player in terms of interplay, dribbling, passing. If we take the view that we can't keep carrying these messy players that don't suit possession football then you can probably justify a sale.
I'd still lean towards giving him another 18 months here and see how it goes, if he's not upped his level and consistency then let him go then. He will still have some value at the end of that time frame. We shouldn't be in the business of selling promising home-grown talent too early in my opinion.