Which is better for monthly DCA into VOO, FSMOne RSP or manually using Tiger Brokers?

Definitely FSMone RSP. FSMOne is 1USD min or 0.08%. While Tiger Brokers is 1.99USD per trade. Unless you're invested so much per month that if you make a RSP on FSMone, the fees are more than 1.99Usd.

If you want the best fees, TD is 0. But account opening is gonna take some time.

Other brokers you can consider is first trade (free) or interactive brokers. But I havent tried these 2, so I cant give any advice.