Be slightly careful here. If you want an EasyMesh connected to your BPI-R4, it is called EasyMesh in AP mode as a Controller and this is only available on some TP-Link routers as this is a very recently introduced feature of EasyMesh.Actually, I was thinking of using a bunch of BPI-R4 with BE14000 wifi7 to DIY an openwrt mesh using wired backhaul and 802.11r. It would certainly be the most work, likely give the most wifi headaches and require the most effort to maintain. And each BPI-R4 is half the price of a retail wifi7 router/AP. Patches will likely be available longest.
Then I thought if i used Starhub as ISP, why not just use their EB810v instead at $360 each. BE22000, commercially supported easymesh tech.
But with your inputs, I should really just buy a bunch of HB710 (SGSH or SGMRP). $192 for BE19000, easymesh and patches. I can use these as APs and run openwrt as the main router to have maximum router config flexibility and supported wifi. (The only problem is that the HB710 only has 10GE copper ports, and I was planning to convert my structured cabling to fiber runs...)
Refer to this document -> TP-Link routers supporting EasyMesh in AP mode
The HB710 is not in that list so I don't think it can be a mesh controller as an AP. Obviously, you can attach several HB710s simply as multiple APs' to your BPI-R4, but they will not function as a mesh, and you will not have the wonder of being able to wander from router to router without your video WhatsApp call disconnecting. Not all is lost however, because the retail Archer BE800 and BE805 are on that list and can be a mesh controller as an AP for a bunch of HB710 satellites. But they are more expensive. Maybe TP-Link will update the firmware on their Aginet routers eventually to support control as an AP.