Turn off your watch history, don't use the YouTube app at all, use a web browser instead, and always start your browsing on
https://www.youtube.com/feed/subscriptions which only shows your subscriptions rather than random other things. To the extent that suggestions still show up alongside videos you watch, uBlock Origin rules can help hide those so you can remember to stick to only the channels you've subscribed to.
Alternatively, YouTube channels have RSS feeds, which only contain the videos from that channel, so using those in a feed reader could help.
