WiFi 6 / RBR850 issues

I have similar setup (apart from the link speed and ISP) and on the face of it what seems to be the same problem.

I recently bought an Orbi RBR852, AX6000 setup (one router and satellite)

I've tried 2 different Intel AX200 based cards (different manufacturers) with various vendor and intel drivers but under certain loads the connection is dropped regularly - Veeam Agent for Backup (to a local NAS, plugged into the satellite) for backup and Wan Killer eventually drops enough packets to cause an issue. Ping shows the connection is solid, then every so often drops 2-3 packets. That seems to be enough for the backup\stress test to fail.

I've also tried the same loads with an ASUS AC88 - this maintains a solid connection, but it's 802.11ac - too slow. Surprisingly, even if I force the ax cards to use 802.11ac, they still drop. 

Router and satellite are about 30 ft apart with a few walls inbetween too. Signal strength between the satellite and PCs is strong (much more than that from PCs to router), so PCs should not be swapping between the 2. There's no serious contention that I can see on WIFI from my neighbours.

For the moment I've plugged both PCs and the NAS into the satellite router and that solves the problems during backups, but this is far from ideal.

FW Version - V3.2.17.12_1.4.14

I'm at the end of my tether with this one. If it's a chipset\driver issue on the cards, I'll take the hit and buy another type of card, but I can't imagine these cards were released with a major flaw like this. Is there a compatibility issue between the ax200 cards and this Orbi setup?

Any suggestions greatly appreciated :)

Scotty