Resize tabs on desktop without affecting Above/Below Left/Right

I agree. I wish each yellow double arrow only controlled the dashed line segment until it intersects another line like how the green arrow does.

It's possible but not totally straightforward and has some limitations.

Split the bottom left frame and now merge the bottom center and the bottom right

You mean, like on Hog4, where you can have everything in its own window with you own size?

Hmmm...