@MarkG3083
I wish I had your service provider. I could use free M5.
I run Deco mesh which consists of two M9s, Main and Satellite with wireless backhaul, also one M5. Usually my M5 is wired, but once I ran it with wireless backhaul for three days, for testing.
During the time M5 was on wireless backhaul, I haven't noticed network speed degradation for devices connected to M9. There should not be, but it is always nice to have theory proven by practice.
Just note that M5 is not as powerful as M9. You will see somewhat slower speeds overall comparing to M9, and difference is more noticeable when M5 uses wireless backhaul.
My suggestions for adding M5 to M9 mesh would be:
1. If you have wired Satellite M9s, there are first candidates to be replaced by M5s. Run M5s wired, move freed M9s elsewhere.
2. If you haven't done it yet, turn off 2.4GHz on Main network and use extra Deco hardware you have to cover places with weak 5GHz signal. The speed difference from running mobile devices on 5GHz only could be quite visible.
3. If you have places where people in your household use only devices like smartphones, tablets, meaning lower bandwidth requirements, you can position M5s there.
4. Desktops, laptops, any other computing hardware that is bandwidth demanding, this is where M9s should be.