That can be a good solution at least if electricity costs are not a big deal. If power is expensive in your area, it might be worth to buy something more power-efficient, like a raspberry pi (assuming they’re not completely sold out right now).
No company is just going to host a server for you for free, a (virtual) server for running nextcloud will cost you at least a few bucks a month. As others have already said, you can run a server at home on your own hardware, but this is also not free (hardware cost, electricity, etc.) and you will additionally have to deal with any hardware issues & replacements yourself.
I think it’s a fairly standard feature. At least Protonmail also supports this kind of “alias”.
This might be what you’re looking for. When translating between languages other than English (German - Spanish in my case) the data can be a bit lacking but it generally works really well.