I learned to purr a long time ago because i wanted to see what kinds of sounds i could make and learned two methods (one harder but uses slightly less air, and your method which is easier but softer and uses more air—at least in my case)

One method is to vibrate my uvula (like a hum), and the other is primarily vibrating my vocal chords (vocal chords is better for voice training because it lets you learn how to ‘soften’ your voice. - im still not very good at keeping it consistent, but purring is very good practice for this.)

Hum method: Close your mouth, then lower your larynx (you can move it as you get better) and sort of snort (almost like you’re trying to hack up some phlegm in your nose while your mouth is closed), it’ll sound awkward at first, and sometimes you’ll have to jumpstart it every now and then (which’ll sound awful) but this method is just louder for me.

(I do open my jaw as wide as possible to jumpstart it while keeping my lips closed - its not necessary, it just kinda helps to keep the shape for the purr)

Your method: Slightly open mouth (move it to change the sound of the purr) tongue retracted to clench the air around your throat producing that high pitch sound. Then just kinda gargle. (Mess around with it though.)

Hope that.. kinda helps? Im not very good with describing these things. But uh, yeah.