How do you use 3rd party apps with Mastercard or Visa gift cards purchased with Amex to pay rent?

I'm sure this has been asked a million times, but I believe many rules have changed recently. New to points/churning and trying to get my head around how to earn Velocity points and meet min. 90 day spend using Amex Vel. premium. Rent is primary expense with which to meet the target. This is currently done via transfer only so require manufactured spending or use of platforms with pricey transaction fees for use with Amex.

Apparently buying GC's with Amex/flybuys to get points, then loading GC's into an app such as Beem can be used but can't figure out how that works after logging into the app and looking at their T&C's. And then there is the activation cost for cards - does this outweigh the benefit and therefore might as well just go with Snipp or similar and accept the fees?

Would like to know how others are managing this specifically with Amex velocity Platinum and avoiding excessive transaction fees. On the verge of starting an excel sheet to compare info but coming across a lot of conflicting info between AI recommendations and actual T&C's and it's doing my head in! Might have to just take the plunge and losses through experimentation.