Ignoring the X239/SL4 which is going to start at least a year later than all of the other routes, I think this is the best route I can work out using current info available:
Start at West Croydon 5:50am, X68/SL6 to Russell Square, walk to TCR, Elizabeth Line to Abbey Wood, any bus to Thamesmead, SL3 to Bromley, SL5 to West Croydon, X26/SL7 to Heathrow Central, X140/SL9 to Harrow, SL10 to North Finchley, SL1 to Walthamstow, SL2 to North Woolwich, 474 to Custom House, Elizabeth Line to Bond Street, Central Line to Shepherd's Bush, 607/SL8 to Uxbridge, finish in Uxbridge.
Obviously there are no timetables for most of the routes yet so I can't work out if this is feasible in one day. The 5:50 start time is assuming the X68/SL6 timetable is the same as now when all of the routes are running.