During Lockdown I’ve tried most of the various Apps - Tacxx, BigRingVR, RGT and Zwift. Only RGT allows uploading your own routes. But, for me it was a faff because you need 2 apps running and you need a subscription.
So, rather than trying to replicate Arran, why not choose a course on Zwift and have a target time/effort to complete it? For example it could be a group ride with a leader who has calculated that to meet the target we will, say, ride at 3w/kg to keep things in order. Happy to get into it in a couple of weeks. Certainly could be something to consider for the winter.