why pay extra money for extension if you are planning for AOS? just get married, and sent your AOS concurrently before your I94 expired, therefore you won't be out of status (if this is what you fear of).
I also doing my AOS from B2 visa since last October 2021. I was just planning to stay for 3 months visiting my bf when my country close it border due to delta got very bad last year. So we decided to get married not even know about the 90 days things (it's just something people said that USCIS will consider its a visa fraud if its too soon from entry?) we got married 14 weeks after i arrived and 2 months after marriage i sent my AOS. I will do my interview next week. I did everything myself without lawyer help.
If your marriage is genuine, if you didn't plan to do visa fraud it's all just change during your visit, i think its fine to do adjustment of status, and you don't need to worry about it.