most "rules" influencers teach you are nonsense. this goes double for when they're preying on your bodily insecurities and telling you what you *must* do for your body type.
unfortunately, a lot of "big and tall" brands are crap quality. you could get something made to measure, but if you're actively losing weight, that might be a waste of money. So your best option will be to thrift. Have stuff tailored if you lose like ~10 pounds. Resell it if you lose like ~30 pounds.
Derek Guy will teach you about proportion: https://x.com/dieworkwear
This article is also very good: https://alittlebitofrest.com/2022/01/09/how-i-think-about-proportions-silhouette/
There are still no hard rules, but these principles tend to work very well, or at least will help you train your eye to understand different silhouettes and effects very well.
These principles are also intended for all men. In the context of tailoring, you'll learn: if you want to look stronger, enhance your chest and shoulders with structure. If your pants are too low in the rise, that shortens your leg and doesn't drape right and makes you look weird. If your pants are too slim at the thighs, there will be a discontinuity between your jacket and your pants, which is bad. Some of these lessons don't apply directly to other stuff -- low rise denim makes sense, short blousons make sense -- but you'll get a hang of that soon, I promise.
Put together these lessons bit by bit to get a greater sense of proportion and get an idea of how to put together a whole silhouette.