I think you missed the point of this thread. The OP has a large industrial lathe (HUGE to be more accurate), which dwarfs all hobby machines. It has a 3 phase 40HP motor. There is no simple solution to power this in a home shop with single phase power.I have been following this discussion with interest and after debugging the variable speed controller of a mini-mill with DC motor I have come to the conclusion that simple is better. A 3 step pulley and belt works well enough for my needs and as for the mini-mill I directly connected leads to the motor to supply 90 VDC in the event that the speed controller failed during an important job that I needed to finish. Luckily I found a few poor solder joints and a defective (not too robust) speed control pot to make the mill run again. For my Bridgeport I have a single phase 240VAC motor and it also suits my needs.
For hobby use I like simple machines although 3D printing needs some investigation.
I do want to correct your assertion that VFD's are complex. They are commodity machines these days, and very affordable. 40 years ago, they were just coming to the HVAC market for fan speed control. Now VFDs are in home washing machines, and air conditioning systems.
Simple is plugging a lathe or mill into a wall outlet as purchased from the tool dealer. Wiring a VFD does take the ability to connect "door bell circuits", as making cords and wire interconnect that operate at AC line voltages and currents takes some domain knowledge to do safely. From reading comments about hobby machinists purchasing industrial equipment, there is this fear of buying a 3 phase machine, as they now must add additional wiring and systems that generate 3 phase, and that may push them into an area where they lack the knowledge. The cost of a rotary phase converter (RPC), purchased off the shelf is moderately expensive. There are many posts about making RPC's home-made. There are many posts about connecting a VFD to a 3 phase motor. If you price a VFD vs RPC, the cost difference is nominally 3:1 less expensive (in the 3HP, 4.5Kw size). There are very cheap VFDs of the Hungyang variety that currently are not reliable (that will change, but gives a bad name to VFDs, and hobbyists have found these $110 USD solutions attractive). Most namebrand VFDs are manufactured in Asia, from quality designs.
Changing a motor to DC and connecting a speed controller is just as "complex" as connecting a VFD. Both give variable speed. If the motor comes from a treadmill, that takes additional skill to make a motor mount as well the skill to hack the electronics.
When I see machine shop equipment sold locally, the 3 phase machines are less expensive over single phase. We all are cost sensitive in this hobby, and passing on a machine because of a perceived "complex" power system needs to be understood by those in the market for shop equipment. If a hobbyist does not have the skill to connect a VFD or RPC, they should stay away from 3 phase machines. But that is a small count of hobbyist's that grew up fixing and manufacturing repair parts, as they also repaired electrical appliances, and learned how to work with AC power.
All that said, my Wells-Index vertical milling machine has a 1960s 3 phase motor. NEMA changed the motor dimensions since this machine was made. That and it has a bastard C-face motor. It would be a major effort to convert to single phase. Originally, I powered it with a home built RPC. This worked but I always had to manually start it, and it was noisy in the shop. I found a very affordable VFD, and I would never go back to fixed frequency operation, as I now can control spindle RPM directly from the VFD display. It makes switching between using an edge finder, drills, boring head, quick and I can convert the SFM to spindle RPM, and dial in the speed to not damage my tooling or the part.