[NiHaoMike]

Single (split) phase VFDs do exist, but 3 phase VFDs are more common and cheaper on the surplus market. If you go that route, get a VFD with a RS232 or RS485 control and an interlock loop for easier control. Connect the RS232 or RS485 to a Raspberry Pi or similar using a USB serial adapter, onboard UART (plus level shifters) or equivalent. The interlock loop is for the high and low pressure controls. (The delay on break is no longer needed since the VFD would act as a soft start.) Put the VFD within a few feet (shorter is better) of the compressor to avoid standing wave issues.

The 20 minute on/off cycle is just to prevent short cycling. It tries to keep the compressor running for at least 20 minutes at a time (only exception being a fault trip or manual command) and also preventit from restarting until at least 20 minutes have passed. That's just an example - you might want to try different times.

Once the discharge temperature gets near the limit, the only thing you can do is lower the condensing temperature. In practice, R290 is very good with keeping discharge temperatures low so I wouldn't expect problems.