Hi, yes, although it can "pair" with other mesh but it will function as wireless bridging & not fully mesh.
If you configure your M5 using wired dedicated backhaul & run your M5 in AP modem then it should not be a problem.
If you wish to turn your M5 into Router mode & have it dial pppoe, then you will need to set HG8145V5 to bridge mode.

There are some confusions even at SIs level, but no worries, just buzz me if you require more assistance.

Thanks.

You can use other brand as AP because HG8145V5 won't be compatible to M5, but I would recommend just switching to your own router. It depends on your budget and the size of your house.

Yes, one connect to router and the other put at another room.
For 1200sqft condo, and to get consistent WiFi signal, you can consider 3-pack Deco M5.
That should solve your issue.