You can buy the one-off membership, but its $59 (https://www.warzone.com/Purchase?ID=9
I agree with you that thats not worth it for a few SP games.
Have you tried Multi Player already? Those games are run on the server, so they are a lot less laggy.
EDIT: You can also make a 2nd account, and create a multi player game (so it runs on the server) with 2 players and 38 AI's. If you then surrender with your alt, you can play with the 38 AI's without any lag (unless the game itself starts lagging out your browser, but if you dont have to run the AI's that should already help a lot).
Edited 2/16/2022 20:40:03