Retrofit - LinuxCNC + EtherCAT

A very common method was to start on Y configuration and then switch to Delta. The motor starter switched between Y and Delta (contactors).
Ask google or chatgpt for the reason.

Depends on what you are aiming at, but a standard chinese VFD controlled by Modbus/USB is a common way to get speed control of the spindle.
The rpms can be somewhere 50%-150% of nominal rpm. Low torque at low speeds, possibly balancing issues at high speed, but at least some variation up and down is possible.
USB/Modbus is somewhat sensitive to electric noise so it is usual to have USB serial/RS485 with optocouplers and line filter for the mains.