I don't follow your comment regarding getting to a weight class. Almost all combat sport gyms could give a shit what your weight is unless your going into a weight controlled competition. There are women, guys who weight 130 lbs and guys who weigh close to 300 at most locals.I've always wanted to, I've always said once I get to the weight class I'm going to join something. I like the gym but I don't like how it's all knowledge and results, and the results disappear so quick when u stop. But an actual sport like MMA or boxing also requires to learn skills which doesn't go away
Once you are approved for sparring - they will set you up with someone of somewhat equal skill and size.
The results from weight lifting if not gained by PED's only, don't disappear - you won't stay the same if you are on a gram of gear for 4 months of multiple PED's and then stop but if you lift weights and want to keep it. Yes, knowledge is key, changes in your lifestyle like diet which is usually a healthy change, even training a few times a week. If you build muscle and lose some after - who cares. Muscle memory is real and if you go to any decent facility for self defence training / MMA or whatever you want to call it - there is usually a component of strength building which is often BW exercises for warm up and to help in your abilities.
If you depend on PED's for all your gains - stop, it's a waste of time. If you use it like a tool in your toolbox to help but is only a small component compared to diet, training and rest. Look at @BanditNOLIMIT 's diet and his avatar - he didn't get there with PED's alone. If it was me I would add in more fruit and vegetables but his diet is consistent and built to support growth. I read his posts as we never stop learning. @Oldguyjiujitsu has tons of posts on diet and is worth a read as he has a great diet IMO. So do many others so not excluding anyone but they come to mind.
Any combat training - at home, in classes, or with individuals you hire won't remain with you like walking does if you don't keep practising. Will the basics remain - sure, but if you don't keep working on foot work, continue to practice even once a week - you will forget and lose much of you learned. Flexibility is essential for most combat sports if you don't like injuries and you will lose that if you stop if you don't keep stretching. My point is, if it helps you in any manner pick one (any of them) - boxing, kickboxing, judo, I hate Jiu jitsu (hate the latter as I found it was easy to get hurt but love learning the chokes). If you want to start by yourself, there are great you-tube videos on proper technique and buy a heavy bag or BOB if you can afford it. Do it long enough and if the situation ever presents itself, you won't have to think - the practice / training just takes over. Further, having a bad day or struggling - a 30-60 minutes session stretching, warming up and smashing a heavy bag or practicing precision on a BOB is therapeutic. Even 'air boxing' is great.
No time like the present - you don't have to walk in an join and be ripped or lose weight before you go in. One the best Judo guys I know and now trains in Krav Magra looks like a cartoon character.
Hope you find something you like as if you don't, you probably won't stick with it.