I use a basic garage fan with a speed controller near me so I can change the speed on the fly. https://www.amazon.com/dp/B07T9BLJ7L?psc=1&ref=ppx_yo2ov_dt_b_product_details

Looks like lots of people selling the same product. Some reviews said there's not much range of control, it's that what you found? I am looking at this one https://www.amazon.com/dp/B07T9BLJ7L/ref=cm_sw_r_cp_apa_i_gMIbFb9FAA4K3

This is what I use. Its not going to tell you exact RPM of course, but you could potentially measure it to determine specific speeds.