Lenovo M72e with an i5, I use one as my test bench. Even works well using a parallel port using software step\direction. Be careful as some may have a G series CPU, but these can be swapped out for a 3rd gen i5.
Odroid x86 H series work well, that what my mill runs with, H3 in my case.
If you want to use a arm based SBC I'd go for a RPi5, images are available and work with no issues.
Honestly the best bang for buck is a ex corporate PC.
With the RK you have to build\source your own RT kernel.
I have seen your thread, have you actually tried running a config with the PC you have ? In a real life situation you are not going to be watching 4K youtube videos when machining a part. Just give it a go and don't get so hung up on latency. With a setup using the Parallel Port it is an issue but an external step\dir generator board shouldn't be so picky about latency nor should it require a base thread, servo thread should be all it requires..
What you may spend on a suitable new PC (especially if you are considering an Rpi5) maybe equal in cost to a Mesa board...and whilst the remora project is well supported and has had a lot of work put into it I'd know which way I would jump.
I've had luck using AUD $40 Spartan 6 dev boards running Hostmot2 firmware using either an SPI or EPP interface. Apart form Mesaflash issues I'm also having success with Ethernet as well.