(Disclaimer: I work at Netgear, but am not speaking for them - these are my personal opinions only).
It's... complicated. Today, Qualcomm still bases its SDK on OpenWRT v15, and supplies its 'secret source' as black boxes pre-compiled for v15. Trying to use those black boxes on anything but the OpenWRT and toolchain they were built for is painful and probably ultimately unfruitful. There will be problems, and Qualcomm won't put effort into 'unsupported configurations' without a very powerful business reason. While this is not a static state of affairs, I can't comment about how it might change.
As a general rule, the Netgear Orbi product line uses Qualcomm silicon, and Nighthawk uses Broadcomm silicon. The R7800 is one of the few Nighthawk routers that's based on Qualcomm silicon. Because the Orbi products use mesh networking, there are more Qualcomm 'black boxes' in their firmware than before. That makes it harder to provide contemporary versions of OpenWRT on recent Qualcomm-based, mesh-enabled hardware.
In terms of memory, some Orbi models have more, but usually lack a USB port, so there's no way to supplement it. The reliability issues you're having with consumer thumb drives are because they're not rated for continuous use as a filesystem. You'll see the same problems if you use a 'normal' SD card on a Raspberry Pi or a security camera - a usage pattern much higher than they were designed for will eventually cause them to fail 'prematurely'. Try to find 'high endurance' flash products, they'll work much better in for that use. Either a 'high endurance' SD card and a USB SD card reader, or perhaps a thumb drive marketed as an 'SSD'. Yes, they're more expensive, but there's good reason for that.
Since WiFi standards continue to evolve apace, I personally prefer to separate my AP from my router, rather than using a combo unit. The same way I prefer to have a separate cable modem than have my router integrated with it.
Personally, I use an access point from Netgear's business range, because
a) it's much less expensive to be swapping just that out when the next WiFi standards roll around,
b) the ceiling mount & PoE support means I can put it in an optimum location, on the ceiling, central in the house, without my router having to be placed there too, and
c) many of the business APs have nice features like multiple SSIDs on separate VLANs you don't see often on non-business oriented products. I like to segregate IoT traffic for example, and being able to dedicate an SSID/VLAN to that makes it so much easier to do.
Anyhow, hope that helps,