I use a 5.6kw spindle with a "7.5kw" HY GT VFD.
On mine it displays HZ when I adjust it and voltage when it's running. I'm not sure if it can be changed. It's definitely annoying but not enough for me to start trying to read their pigeon-English manuals again...
They aren't the best VFDs or even good but they work and are the cheapest. That is the only reason to get one.
You have to go up a a size or two even though they advertise 1ph in / 3ph out, so you can't use their 7.5kw vfd for a 7.5kw spindle.
Unless your 2.2kw spindles are stalling, I wouldn't recommend going up in power. Upgrading from a 2.2kw 24,000rpm to a 4.5kw 24,000rpm spindle won't allow you to use large tools or work well at low speed etc.
You may be able to take slightly more aggressive cuts in wood. If the aim is to use 3/4"+ tools in aluminum then you're better off adding a second belt drive spindle with a gear reduction.
Or, upgrading to a 2.2kw spindle that reaches full power at 6000rpm like this:
https://www.damencnc.com/en/quicktoo...rpm/a3969?c=22
It has way more low speed torque than my 5.6kw spindle without wasting power when using small tools at high speed.
My spindle uses an electric fan. It works fine for all day use as long as I cut aluminum with coolant. It's better than a Chinese shaft fan spindle as it cools even at lower speed. Water cooling is still better though.