How much accuracy do you need on the final machine? Somewhere between 0.01 - 0.05 for general hobby might be OK and was the range I was aiming for on my latest machine.

If you look at my YouTube video around episode 16 you can see my experiences using epoxy to level rails directly (channel is routercnc)
I found it set level to at least 0.05 mm and probably more but I couldn’t be sure as my reference surface is not a proper calibrated one. Just be aware of the problem with bubbles and ways to get rid of them.

If you want to level rails you can also use shims and an engineers precision level. The 300mm square level was around £100 but measures down to 0.02mm over 1000 mm. I must have paid ~£80 for 2 sets of epoxy mix so actually the level is good value as it can be reused !