Thanks
@DaveR.
Just to close the loop for all on this thread, based on advice from a different forum, I downgraded the firmware to a previous version (4095). It seemed to work better for a day or two, with the 5GHZ-2 backhaul for most links (except one router that is a bit farther away - makes sense since 2.4 GHz penetrates walls better). But then it reverted back to its previous behavior. All backhauls reverted to 2.4GHz. The download speeds were terrible - I have fiber optic with 500-700 mbps download and ~300 upload at the main router, but in rooms more than 1 room away, speeds dropped to about 20 mbps download, 4-5 mbps upload, the same as my old DSL. And routers kept dropping out at strange moments.
So I gave up...I returned 4 routers to Amazon for a full refund (thanks Amazon!! you may treat your employees like crap but your return policy is great), and one (bought on Amazon Marketplace from a 3rd party vendor, not so friendly in its exchange policy) I sold at a decent price on LeBonCoin (French eBay).
Meanwhile I bought the Linksys Velop mesh system for about half the price (
Linksys Système WiFi Mesh Multiroom triple bande Velop WHW0303 (routeur Wi-Fi AC2200 / extension Wi-Fi pour une portée de signal jusqu'à 525 m2, co ), and it's been great. Download/upload speeds in those more distant rooms are generally 100-120 mbps, and in the adjoining rooms to the main router they're at 200+ mbps. It's gotten faster and faster in the past couple of weeks and is incredibly stable. Never a dropped router.
Lesson learned. Sometimes the priciest, fanciest systems aren't actually the best...maybe they're buggy because they try to do too much...