Do you mean to say “if you’re NOT going to use serialisable”? I think you missed the NOT and every reply seemed to think you were arguing a different point but your description about not using foreign keys and using Redis instead only makes sense if there’s a NOT there.
No. If you're going to use serialisable you might as well just use something like Redis (which achieves serialisable behaviour by the much simpler approach of... actually executing your operations serially, and generally outperforms a traditional RDBMS set to serialisable transaction isolation). If you're going to use the horrendously complex machinery of a traditional RDBMS, it should be because you need a level of performance not achievable under serialisable isolation level.