First Time Grading Advice

Looking to make my first grading submission. Narrowing it down to 15 vintage cards and 10 modern cards that I’m working through via AI graders, card centering app, and forum advice. Questions:

1. Should I do PSA for all? Or are other graders like Becker better for vintage and I should do multiple? I want to maximize investment value.
2. If PSA, should I trust game stop? Or is the yearly membership worth it given I have a good number of cards in the work and will probably do more?
3. What are best first time grader discounts?
4. What are your personal must follow rules when you submit?

Any other advice is welcome! Appreciate the kindness this community has shown in all the threads