I think to get the national 500 grt master without any of the stcw requirements you only need advanced firefighting and radar observer however to take advanced firefighting you need basic before hand. you could go that route and worry about the prerequisites for stcw later down the road when you can. I am sure someone will correct me if I am wrong.
the 200 grt would be easier to get but it will still limit you. having the 500 grt will make it easier to get a MOT if you want to end up on tugs. I went 100 to 200 to 500 to 1600 and I wish in hindsight I went straight to 500 from 100.
See 46 CFR 11.201(j)(1): j) Professional examination. (1) When the Coast Guard finds the applicant’s experience and training to be satisfactory, and the applicant is eligible in all other respects, the Coast Guard will authorize examination… [emphasis added]
This means you can’t sit for the exam until you meet all the course and se a time requirements.
If you were to apply for Master 200, you will still need to show sea service on appropriate tonnage vessels, and there would be at least a limited exam that covers the things that are tested for Master 200 and not for Master 100.
You should not need “advanced” first aid. For a national endorsement (license) without STCW, it’s a one time requirement for first aid (basic) and CPR, so if you have Master 100, you don’t need to re-take this.