The Deco X50 uses Deco Mesh, which is incompatible with Easy Mesh on the HB710 so you can never create a single mesh of the whole network. You might want to consider the $360 EB810v from Starhub, which has 1 10GbE LAN port and three 2.5GbE LAN ports. You can then run the X50 mesh off the EB810v's LAN ports until you decide to replace them with HB710s running Easy Mesh. Otherwise you won't get much benefit off the X50 satellites (other than going from M1 500mbps to 1Gbps) from the 5Gbps plan. Getting a HB710 and a switch is slightly cheaper, but if you do, get a 2.5GbE switch, which is not expensive. No point getting a 1GbE switch at this point.
There should be no issue running two separate Mesh networks as you upgrade slowly from Deco Mesh to Easy Mesh except that when you roam from Deco Mesh to Easy Mesh, you may face a sharp transition. For example, if you are on a WhatsApp video call while walking to another floor with a different mesh, it may cut off.
The ideal network for you is a Deco Mesh of three Deco BE85 routers, which will cost $2k or three ASUS BQ16, which will cost $2.5k. So getting three HB710s for $576 is a super bargain, since it will give you everything those expensive meshes have including 10Gbps backhaul. Thank you Starhub and TP-Link
I have a large Easy Mesh and it works extremely well. I cheaped out a bit and bought TP-Link BE230s from Carousell for $100. I also have one HB710.