No, that's not how it works. Selling Garnacho for £50M would free up £50M in space on this year's books. You're right that £250M in inbound transfers could be amortized across five year contract, but the problem is selling Garnacho doesn't create £50M extra room for the next five seasons! It only does in year 1.
This is what all the people hawking the "youth academy products are pure profit" line are missing. It really doesn't matter who you sell.
Take Holjund for example. If we sold him for £50M, we'd have less room than Garnacho for 24-25 because Holjund still has his previous transfer amortization from buying him. But the sale would create additional room in 25-26, and 26-27, because the previous amortization would be gone in those seasons. Overall, selling Holjund for £50M would create £50M in space, just like Garnacho. That £50M would just be spread out over four windows (this Jan, summer 2025, Jan 2026, summer 2026) instead of one.
Every pound counts the same for PSR purposes. You don't get extra profit for homegrown players, that's a myth. You just get access to spend the money earlier. We shouldn't be thinking on one season timelines though, and hence there is no particular advantage to selling Garnacho over other players.