The question asks what's the best way to get back into shape for Taekwondo after an extended period of inactivity in Taekwondo. Also, what to do about muscle that has been gained since then.
While I do think some people benefit from working by themselves to get themselves back in shape before returning to active classes, I think it's almost always better to just dive right back in to regular classes. That goes for pretty much all martial arts. You'll make better progress by not delaying and just getting back into class. You can always do stuff outside of classes, too. So you can do both.
There's no reason to delay returning to class. And I find that people who do delay often never go back, just because they make such a big deal out of trying to be as good as they used to be before going back. They put such a high bar on their return that they never make it back. The ironic thing is that they'll actually return to their original condition faster if they go back to class and go through the normal training like they used to do. It will all come back.
Part of this is having a teacher or multiple teachers and students who will give you feedback. They'll see what you need to work on, and they'll work on it with you in a way that makes sense for Taekwondo.
The fact that you gained muscle is not a big deal. Your body is different now. You can't go back to the body you once had. You have to go forward with the body you do have.
Strength does not make you slower. That's a myth. It's actually the opposite. You will be faster. But you need to train to be faster. You do that by going to class and working on increasing your fast twitch muscle response. Don't know what that is? Don't worry about it, because it's built into Taekwondo training. Taekwondo relies on this quite a bit.
There are some down-sides to having extra muscle mass. It will reduce stamina. That means that your body will be using more oxygen and blowing through more muscle glycogen (fuel) than before. You'll feel like you tire a bit faster now. And then you can slow down as a result.
To a certain degree, having more muscle mass is generally a net benefit in most martial arts. Taekwondo relies a lot on cardio endurance and stamina, however. So you're going to have to work harder to condition yourself for cardio stamina with more muscle mass than with less muscle mass. It is perhaps why most Taekwondo black belts are skinny looking and have less muscle mass relative to many other martial arts.
Outside of class, to get flexibility back, you know what to do. You have high kicks, leg raises, splits, and so on. Practice those 3 times a day. Do it 15 minutes in the morning, 15 minutes in the afternoon, and 15 minutes at night.
As for muscle memory and regaining proper technique, I'd say don't bother with training on your own for that. You're going to want to work on that in class under the watchful eye of a good instructor. They'll be able to notice things about your technique that you can't see. If you drill bad technique, you're just going to have to work harder to undo that training.
Bottom line: Get back to class. Take 6 months to get back into the swing of things. Don't judge yourself too harshly during this time. After that, you can work on pushing yourself further and working on the things you need work on. Don't compare yourself to the way you were in the past. Your body has changed. You've gotten more muscle, and you're older now. It's not healthy to hold yourself up to the way you once were. Work with what you have now and go from there.
Hope that helps.