I'm about 18 hours deep into a single sandbox game - having the exact same problem. Entertainment has consistently been my lowest stat for like 35 in-game years :/

Having just finally got it up a bit, all I can advise is volume.

I basically just went nuts and placed Taverns absolutely everywhere where they'd have 80+ effectiveness . then sprinkled in 4-6 new restaurants I got free as objective rewards. Both are cheap to build and quick-build, so only small improvements but low cost and fast impact.

idk what time period you're in, but if you've got powerstations and tourism unlocked; spam all your coastlines with beach resorts/clubs (cant remember the name, the little pier board walk things with 1 or 2 huts on them) Build a few hang glider clubs next to them. Add a few cabarets into built up areas and if you have spare power, 2-3 casinos.

I could be wrong, but I think if all your entertainment venues are permanently pretty much full, that means you can build another one pretty close by and still get benefit from it.

Good luck.