pay monthly at SS or online (register at SS) but to fill documents each month. It must be in Thai, your Thai gf can help you with this since you don't have exact many staffs. RD can pay monthly online or you can go directly to office ask for precedure. Good way to learn. It may be tough at starting. Keep monthly pay for annual statement
If you don't have running business or not many transaction in a month. I don't see monthly accountant cost necessary. You can do it by yourself. But in order to continue non B you need 4 thais in company showing proof by receipt of SS and RD. If you don't want to pay to hire then 2,500 is okay choice except you will find some Thai name and put that in your company instead of using accountant service
As a Thai, Most modern Thai woman I know , the educated woman, mostly don't even care about sinsot (they said it's hard to find good heart person already why to require more). Me included. Mostly it's elder in family that asked for sinsot. The understanding family will never asked for dowry but would ask a man to take care of their daughter well.
In Thailand, Thai people as Buddhist mostly are taught to give and donate to other so that person will receive merit after death. And also when they donate to these group of volunteer(as foundation). They can get benefit of double expense in accounting