Happy Drumiversary my friend! Lol.
As far as technique goes, the style that you choose (slide or heel-toe) is going to depend on the genre that you play.
If you need power and speed, I would recommend slide technique. It's taught on this website, at the link below.
However, if you need finesse and softer kicks, I would use heel-toe. This is not a technique I have practiced, and therefor do not teach. It's a lot trickier to get down, and in my opinion lacks the power that many drummers need to play modern genres of music. It's also a lot easier to get wrong, and find yourself in a worse position that when you started. (Like being off balance on your throne, for example, which can lead to hip problems.)
I don't think that your slow doubles on the kick are a speed problem. I believe you have a CONTROL problem, which is quite different.
My advice is to work through a lesson like the Kick Expansion 102 Masterclass, which I have linked below.
This lesson is ALL about doubles with your foot, but it's not focused on speed. In your case, I would find the "breaking tempo" where these exercises start to becoming challenging for you. Let's say that you can play these exercises at 70BPM, but at 75BPM they become sloppy.
Your goal is to increase the speed, moving 1 or 2 BPM at a time. You might need to spend 1 or 2 hours at 71BPM. Then another few hours at 73BPM. By the end of the week, 75BPM isn't as hard as it used to be... THAT is what speed increase looks like.
You aren't doing anything wrong simply because you can't yet play the doubles fast. The issue is merely that your foot doesn't yet have the control to execute the doubles. The solution is slow down and give your foot a chance to learn the motion. Your foot is a slow learner compared to your brain, and this is what can cause some frustration. Your ears can hear the fast doubles before your foot can. Slow down and control your practice, and your foot will catch up in no time!