Likely a lot prior to that, we will get stuff working on arm properly once Apple finish the transition to the A14 or A15 on laptops and desktops.
We want to support dev environments on Macs and Macs are poised to be the absolute fastest single core computers out there now that the latest iPhone is scoring 200 on speedometer.
My estimate is that within 3 years Discourse will work fine on arm dev environments.