You can't teach footwork, balance or preparation? That's a new one on me. Taught those all the time to my students (back when I was a coach) with great success, especially some elite juniors.
You might be one of the many people that think they can be taught by just watching a video. I've seen a lot of types that just feed on tennis tips but never really engrain it in muscle memory, they try it for 10 minutes and think the footwork/stroke whatever is fixed. Just knowing something is half the battle, there's a gulf between knowing and doing on a tennis court. That's sometimes where a (good) coach comes in.
I used to do a cardio tennis class, I think that was great for adult rec players footwork. Unfortunately at times I was seen as a little bit of a task master in that class. It's a very delicate situation to teach some non-athletic adult rec players. Being someone that was pushed very hard by my coaches I had to learn that as a coach. A lot of older rec players have limitations that don't allow them to move very well. If that's the case then overcoming those limitations may be the first step. Or maybe they can't be overcome but there's still ways to get better in increments.
Getting better at footwork is more about doing than knowing is my biggest point. Most people dont want to practice it because it's too much work, they're their to have fun with the very little time they have outside of work. I get it. Different strokes. Most people also just want a lesson where they get to hit every type of shot and get some boilerplate tips (it's fun and it's the equivalent of a practice session but it wont get your game better long term). I could go into more detail about how to get better footwork, but it's really not all that complicated and I'm sure there's plenty of videos online. I like a ladder for getting better at small steps.
For racquet prep I generally taught that two different ways. Simplified version for beginners with just one step. Two steps for more advanced players (stalking phase and racquet back phase). I learned mostly from Tennisplayer.net (John Yandell) not Youtube videos. I tried to create progressions for these things and explain it as simply as possible (no biomechanics terms). Changing a stroke is a sales process, have to sell the person on how it improves their game (I found that quite fun and easy because I knew it would have massive improvements on their game). I can't sell people the same way on here which is fine (showing someone is leagues and leagues further apart than telling them), this place is sometimes more like an auction where the highest bidder (best advice) is whoever fits the OPs bias the best.
For you, Shroud, it could be that changing your racquet prep killed your stretch shorten cycle. That's why theres a stalking phase and right when the ball hits the court on your side you coil. If you stretch the muscles by coiling too early you lose all the power. Hard to say really but it's my best guess. You like many others on this forum will probably not find the answers your seeking on this forum. Because the answer to your question lies in actually practicing the technique and having someone qualified walk you through any questions you have. You'll have to be sold on why it makes your game better and that AHA moment will hit you so hard and your face will light up (one of the great joys of being a coach is seeing this). Having good racquet prep will undoubtedly improve your game.
Also you stated you only gave it one hitting session. Even if you're doing the technique right it will mess up your timing and, yes, you'll have a terrible hitting session. You can't change a technique in a hitting session. Rent a ball machine at a club and just isolate the shot. It will require weeks of practice, thousands of shots. When all said and done the transformation will amaze you. Much better than instant gratification, enjoy the process.
Here's a good video on racquet prep, similar to how I used break it down as a coach, I even used video analysis on my phone: