But if I use just Tor, then my ISP knows that I'm using Tor
Not necessarily, because you can use
bridges. These are Tor relays that aren't publicly known, as with most Tor nodes, so they help you circumvent censorship. Although, completely hiding that information from your ISP is not really possible, because you have to install somehow Tor (by visiting the clearnet), and even if you do this anonymously, nobody can guarantee you bridges aren't honeypots.
You have to keep in mind that your ISP may has a list of Tor Bridges and if you connect one of them, they'l ltag you. In order to hide your Tor usage, you have to use unknown and/or very secret bridge. This Bridge shouldn't be used only by you but it should be used by some other people too but it should be in a group secretly, not an easy task but possible. One can set up an obfs4 bridge on VPS and share it with some friends who won't make it publicly available on Reddit and social networks.
Btw there is a short normal (okayish) WIKI if anyone is interested:
https://gitlab.torproject.org/legacy/trac/-/wikis/doc/TorPlusVPN