Just to clarify, running the machine on 480V with a transformer from a 240V service will only reduce the amps at the machine, the amp draw from your shop service will remain the same. A 20 HP 3 phase motor is going to put about a 70 amp load at full rated power on a single phase service. Starting load is going to be higher than that.
Your only real option is to derate the spindle. The only way I know to do this is to use to VFD to current limit. The other option would be to replace the spindle motor with one of lower HP. I had to do this with my router, it had a 12 HP spindle and I went down to 2.5 HP.
A 5 HP single phase motor (compressor) will draw about 23 amps at 240V. As long as your service is single phase 240V, the service is still going to see a 23 amp draw even if you are running a 3 phase, 5 HP motor with a VFD.
Another option might to be get a 30 KW, 3 phase generator. I saw one the other day for $3000, I've thought about doing this.