…everything.
As of this moment, regardless of linux support, I'm not aware of any USB WLAN card capable of 802.11ax on the market.
This is only partially surprising, once you look deeper into the intricacies of USB wireless cards and the requirements of 802.11ax. USB wireless cards are usually only an afterthought, take an existing PCIe/ SDIO device, combine it with a deeply embedded SOC providing a faux PCIe port to this PCIe (or SSB, BCMA, AHB, …) based wireless chipset and dealing to interface via USB to a host computer (which usually entails pushing large parts of the driver into the firmware running on this SOC). 802.11ax (actually already 802.11ac to a lesser extent) is now based quite aggressively on (Mu-)MIMO to gain a performance advantage, which in turn depends on multiple antennas and a certain minimum distance between them (otherwise MIMO can't work), given the typically size constraints for USB devices (smaller is better) this poses a rather fundamental problem. Another problem is power consumption and the resulting heat dissipation, the former of which is an actual problem for USB (needs a much more complicated power design, making use of the alternative voltage options of USB >=3.0) - the later an even bigger issue for the desired form-factor of these devices (quite a few 802.11ax routers need active cooling, on top of already shipping in quite spacious cases and with bulky aluminium/ copper coolers on the wireless chipset). While I don't doubt that some vendor will start selling USB wireless cards in the future, ignoring the problems above (considering 802.11ax merely as part of the buzzword bingo, "what, you want actual wifi6 performance as well?!") - I haven't seen any so far. One reason for this notable absence of early USB WLAN cards (which actually already started with 802.11ac) is probably that there is only little necessity to retrofit devices with wireless (some sort of it), as mentioned above actually improving throughput (stability, reliability) over the already included wireless support via USB is hard (be it because the device in question wouldn't really profit from the newer wireless standard (host SOC too slow), external addon card too bulky, power requirements, …).
The only semi-supported 802.11ac wireless chipset with USB options seems to be Mediatek with their mt7601u (and later) chipset, but I don't have personal experiences with these devices (size/ antenna/ heat issues are generic to this product class, independent of the chipset). Technically there would also be ath10k_usb, but the comments inside its kernel module don't really instill much hope (and device availability is more than scarce). Obviously there is a plethora of realtek/ realsil USB wireless cards, with the known driver issues you've mentioned yourself.
While one could look as far back as 802.11n (and 802.11g) for the golden age of USB wireless chipsets, I wouldn't subscribe to that point of view either. Even the ideal combination (FOSS driver and FOSS firmware) of ar9170 (carl9170) or ar9271/ ar7010 (ath9k_htc) wasn't as reliable as one would hope and came with their own set of problems (among the inherent ones of size/ antenna/ heat also very limited on-device ressources, RAM, which results in quite unfortunate limitations).
IMHO, if PCIe isn't an option (and even there you won't find anything for 802.ax[0] either), outsourcing the functionality to an external AP (ideally with a future chance of OpenWrt support) should be considered, which will perform better and is usually even cheaper.
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[0] yes, qca9075 and qca6390 technically exist, but devices are expensive (>200 USD) and hard to purchase (lead times in excess of 6 months), mt7915e/ mt7921e cards are even harder to get hold of - unless you can get by with STA operations exclusively, which can be done by an abundance of rather cheap Intel ax200/ ax210 M.2 cards (PCIe adapters exist).
what do I have to give up to find success? Is there a good expensive option? Or an N card?
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