In summary, you're attempting to regain some kicking ability that you once had but have lost due to inactivity and lack of practice. Your goals are to improve range of motion and flexibility, speed, and control even when holding your kick in place without moving at all. And you'd like to even surpass your abilities when you were once in regular practice. Yet, your schedule doesn't allow for taking classes, and you are only able to train in solo. You're asking for advice on how to approach this.
What you described is something very common in martial arts. When we're young and have the time and passion to dedicate to training, we find ourselves continual improving. It's a big part of our lives. Then at some point in time, our priorities change. It might be school work, maybe you have a new career job and have to devote 60-80 hours a week on that, maybe you just have a "job" job that has unusual hours, or maybe you now have a new child you have to take care of 24/7.
All of those come up in our lives as we get older. The first realization is that it's usually not the case that you don't have the time to train. Instead, it's that you've re-prioritized the things in your life so that training isn't a priority.
That may or may not apply in your case. I just thought I'd mention it. Most people have things that they're not able to get to in life. Stuff never gets done, or things deteriorate, like martial arts abilities. And they look at it and feel bad or get stressed out about it. Well, my point is: Don't feel bad. It's not that you don't have the time. It's that you haven't prioritized it as highly as other things in your life. Be happy that the things you have prioritized are getting done. Or, re-prioritize it so that those other things will get done now.
That being said, you wanted advice on how to approach getting more flexibility in your kick so that you can kick higher as well as getting more control over your kick so that you can essentially hold it still at any point in the kick.
You mentioned using speed to gain that flexibility. What you're talking about are ballistic stretch kicks. In other words, you can get the flexibility back if you just kick really fast and aim high, so it sort of forces your flexibility to return.
But Ballistic Stretching has been shown to cause injury. It's because you're attempting to force your tight muscle to extend way past its limit. It works better when you're properly warmed up, but even then it can cause injury. And even if you don't end up hurt, you probably won't be able to make long-term gains in flexibility with ballistic stretches.
A far better approach is Dynamic Stretching. Here, you'll be careful to increase your leg extension gradually. You're going to use the momentum of the kick to just approach the limit of your flexibility, just to where it feels tight but no more. This is similar to Ballistic stretching, because you're using your kick's momentum to stretch, but unlike Ballistic stretching, you're not going past where you feel tight. This will improve your flexibility in the long term.
There is also Static Stretching, which has been shown to improve flexibility also. With static stretches, you will attempt to slowly extend your leg until it just reaches the limit of your flexibility, and you hold it there while relaxing your muscles. There is no movement or momentum involved. When you feel the tightness dissipate, you can try to push it a little more, and hold it there. You shouldn't try to force it. It's just to the point where you feel a little tight. You don't increase the extension until the tightness lessens.
You should attempt to combine Dynamic and Static stretching to make solid gains in flexibility.
Another thing to consider is Isometric stretching. This applies especially to kicking, and it at least partially deals with your other requirement, which is to control your kick even when you move slowly.
With Isometric stretching, you apply resistance in the opposite direction as the stretch. So if you're attempting to kick high, you might practice this by extending your leg out to a place on the wall or on some platform, and then try to keep your leg fully stretched to the limit of your flexibility while also pulling your leg downwards, against the platform or wall. It should feel like you're squeezing and pushing your legs together.
What good does that do? Well, what actually happens during an Isometric Stretch is that some of your muscle fibers that would normally not be extended to their limit will now be extended to their limit. It allows you to get more out of the stretch, basically. And that can have the effect of increasing your kick's range.
Keep in mind that during an Isometric stretch, you should maintain your stretch to the limit of your flexibility while also attempting to add resistance. It's the addition of resistance that engages those extra bits of muscle fiber that wouldn't normally be engaged if you're just relaxing your muscle while you stretch. And that will help you with being able to hold your kick in place or control it even when moving slowly.
Anyway, so that is my recommendation. Work on all 3 types of stretching. Go to your limit, but not further (not Ballistic). Yes, your kicks won't be super high to begin with, and this might make you feel bad. Don't let that get you down. Take heart that you'll be making "real" improvement in your flexibility, not the fake kind that only looks like you're more flexible.
One other thing on flexibility: Hip and groin flexibility rules all of your kicks. There are a number of stretches to help with it. Look them up.
As for being able to hold your kick at any point in its execution or being able to kick very slowly and keep it under control, Isometric Stretches will definitely help a lot. But you should also build strength. The muscles you'll want to work on primarily are: the adductors, abductors, glutes, and core (obliques, abs, and lower back). Secondary muscle groups of importance: quads and hamstring.
You could just keep repeating your kicks, along with the stretching that I mentioned. That will help build some of the strength, but not all and not very much. You're going to need to add resistance of some sort to go further than that. A kicking bag would help. Weightlifting will help. Isometric resistance helps. Etc.
Aside from that, there's just the time factor and regular practice. That's probably the single most important thing here: A daily stretch routine is a must. In fact, I'd say you need to stretch at least twice a day. Make sure you warm up first.
You will definitely improve if you make it a priority!
Hope that helps.