I plan to selfhost nextcloud, for now just for bookmark sync. is there a point to installing a vpn on the computer running the instance? it shouldnt matter as long as i have https right? what about if i dont have a domain? i cant have https without a domain (ill buy one later just want everything to work first). or maybe use one of those free domain providers for now to get https? what do you guys think?
It's an extra layer of security. Your nextcloud instance won't be reachable from anybody who isn't on your vpn, if a bug which allows unauthorized access gets discovered you will be protected, if they steal your nextcloud credentials you will be protected, but if you're on a device without the vpn you won't be able to access nextcloud.
As for the domain you can buy a random xyz for a couple of bucks per year so just do it
Sure, you can use a VPN if you want to spin up the instance and connect to it without having a domain. You can always open the instance of Nextcloud to the internet later, when you buy a domain.
Get a cheap .XYZ domain if you just want to experiment with spinning up a reverse proxy.