My default response is BTT E3 mini v3.0 but since you're looking to add more steppers something like a BTT Manta might be the way to go. Another option with Klipper is you can use multiple boards, I've only looked into this but you can use something like the E3 mini running Kilpper and then use the stock creality board to run up to 4 other steppers plus whatever. You're going to need a system running Linux for Klipper either a raspberry pi, a VM, or just a cheap PC you're not using.

I'd take this opportunity to upgrade to a Klipper setup. Find which board fits the Ender 3 Pro that can be flashed with the Klipper firmware, then get an external klipper controller, go from there.

I can personally really recommend the fly e3 pro v3. It’s well priced and is a 32 bit board that comes with both an stm32 and a esp32 built in allowing you to run duet reprap firmware without a spare raspberry pi thanks to an amazing port. Since it’s reprap firmware you get all the advantages of klipper like web gui, input shaper, editing config files without recompiling etc but without having a spare and quite expensive raspberry just lying around. https://s.click.aliexpress.com/e/_DCEqwdT

https://teamgloomy.github.io/fly_e3_pro_v3_general.html

I stand by my decision to use a duetwifi.