"Other than the heel toe" - what do you mean? Can you do it already? Or don't you like how it feels? Or the sound, as you might not be able to get the 2nd stroke as even as the 1st? Do you imply the 'constant release' technique with heel toe, too? In a way, they're similar but with the constant release approach, the ball of the foot would stay on the footplate all the time and the heel doesn't need to have any contact with the pedal.
Also, keep in mind that with both heel toe and constant release, there's two ways to do it: 'interlaced' (heel/R, heel/L, toe/R, toe/L, etc.) and 'straight' (as I call it - heel/R, toe/R, heel/L, toe/L). They feel very different but I always recommend to learn them both, or at least experiment with them.
Then there's regular singles. Some drummers never bother to learn anything more than singles for their footwork and their feet are amazing! It depends on the speed you want to hit. Doubles should make it easier to hit extreme speed. And then there's swivel which (in my case) takes quite some time learning but I feel it's an even more ergonomic way of hitting/maintaining high speed.
That's the 'technique' side. Then there's the rudiment/pattern aspect. No matter which technique you're using, it's quite a difference to play continuous 16th notes vs. broken patterns or short bursts. 16ths are way easier. Slow down for broken up patterns, address a few of them one at a time (e.g. groups of 3, 4, 5 notes). Try to have your feet rest on the footplate whenever you don't need them being active.
Do a search for foot techniques and double bass playing to find tons of existing threads.
As mentioned, Stick Control is superb. But you need to put in a substantial amount of time. I'd recommend learning the single stroke roll and double stroke roll first. Anything else is merely a combination of those - but not necessarily easy. Then start mixing singles and doubles, e.g. after 2 or 4 bars, then after 1 bar, 1/2 bar etc. The quicker you mix them the harder it gets.
The paradiddle would be a great 'short' combination of singles and doubles. This rudiment will open up the door for a lot more stuff (hard to get the notes even though).