RE investor has just passed the RE exam. Question about the fee

[Wai Chan]

Hello, I have been a RE investor since 2014 (Own 8 SFRs in DFW and 3 triplex in San Diego) I have just passed the RE exam. and plan to use my license to do my own deal or friends. I have found a one guy brokerage who is fine with me hanging my license under him and won't charge me anything (No monthly fee, no commission split). However he told me that I need to pay around US1300/yr to join different realtor association and to get MLS access and the Sentri.

Sorry for all the dumb questions...:

1. Is it compulsory to join those realtor association as a realtor? 

2. I have been using redfin to look at the deals and I feel like the interface is even better than the MLS page that my previous realtor has sent me. What are other benefit to get MLS access?

3. Sentri key is the most important thing I want to get. Can I skip the two things above and just get the Sentri access? Does it have any dependence on other stuff that I need pay?

Just trying to keep the expense low given that I don't think I will do transaction actively with my license...