Good, easy to flash, cheap and fast wifi5 routers are quickly disappearing from the market (e.g. the r7800 is no longer in production, the higher priced (QoS for gamers)/ newer (but hardware-side basically identical) xr500 seems to have just left production as well). For the most part, the devices meeting your sqm performance requirements have been replaced by their wifi6 successors - because there is no commercial market for high-end wifi5 devices (and their prices) anymore; wifi6 has entered the low-cost market and wifi7 has its foot on the doorstep. Even the first generation of wifi6 devices (e.g. mt7622bv+mt7915) is getting replaced by the (cost- and feature) optimized second wave (e.g. filogic 820/ 830). What you end up with, are either cheap low-end devices (e.g. mt7621a+mt7915DBDC) which have no chance of meeting your performance requirements - or devices that are simply too new for your 23.05.2 support requirement (and yes, also exceed your price cap and often the ease-of-flashing one as well).
EDIT:
Why are contemporary devices often harder to flash? Basically for three reasons:
- they may be too new for someone to have figured out the details of the OEM upgrade process
- regulatory bodies on both sides of the Atlantic are insisting on the regdom being hard-locked - and the easiest way to ensure that, is by locking down the whole firmware (including signed firmwares and secure boot)
- vendors are expected to foot the bill for failed 3rd party firmware installations and have to deal with fraudulent RMA attempts (see just the last 24h for such a case, of someone force-flashing the wrong firmware of another device to their unsupported router, but expecting a warranty replacement from the seller/ vendor nevertheless)