Here's a conundrum for you

Fat floats, muscle sinks, if you bulk up the muscle groups then they are not fit they are just weight lifting effective and have little fat so they sink.
Nothing builds paddle fitness like paddling, if you can find a land based, paddle equivalent then do it in long and short bursts, like interval training, but with your arms and not your legs.
I would look seriously at two things and compare them.
Your gym program and what is offered in the fitness part of this forum and figure out how to do the two.
Most really good surfers have a low bulk and a low fat ratio and are whippet like paddling machines so swimming too.
OH and BTW a few go outs in weak onshore slop and paddling against that will help too.
I read all your other posts, one thing stands out amongst it all, you are trying to paddle a board that is just too small for you and you are doing everything except getting a bigger board.
That my friend is called,
arguing to support your limitations.
I've taken up troll hunting just for fun, instead of a rifle I'll just use a pun! 冲浪爷爷