If you are finding that NextCloud is using too many resources, and just packs way more functionality than you need just for file syncing and sharing, then OpenCloud may be the answer.
You are not going to have calendar, mail, chat, etc functionality with OpenCloud, but it also does not load a bunch of other services that NextCloud requires to run.
It does have file sharing with passwords and expiring access, and interestingly it handles some basic tasks like PDF viewing on the client side. So this will happily run on a Raspberry Pi.
See https://www.xda-developers.com/opencloud-lightweight-nextcloud-alternative