A traditional plastic router is pretty much always the cheaper option, as you get two good quality wireless cards and a 4+1 port 1000BASE-T managed switch 'for free' as part of the deal.
Where these traditional routers (even the high-end ones) fail, is towards the higher end of the spectrum, if you need to cope with 500 MBit/s WAN speeds or more, but in those cases don't ignore x86_64 either - and for all practical intents and purposes you'll need one (or more-) of the aforementioned plastic routers in addition to take care of the wireless side (half-decent mini-PCIe/ M.2 WLAN cards are expensive, barely any 'small' boards take two of them (three for wifi 6e), even trying will be more expensive than a high-end plastic router; don't even think about USB for wireless), …and a dedicated managed switch.
On top of this, not all SBCs with (computationally) fast ARM cores are fast in terms of I/O throughput (best counter example would be the RPi3 and earlier, which are hampered by their anemic USB2 system bus needed for the ethernet ports), they're not a safe bet by themselves, you'd need explicit confirmation about the I/O performance.