Oh, I think under the circumstances, that RG-58 isn't all that bad.
May not have the lowest losses possible, but certainly not complete
'crap'. Replacing it with something 'better' is certainly an option,
but not the first one I'd try.
Options.
Try all that you can think of and see what happens. If things are
'better' on one particular band doing it 'this' way, then I think I'd
note that when using that particular band. Repeat for the other
bands. Lots of work? Sure, but if it works, it works. Finding one
antenna that does it all is almost impossible. Too many variables
depending on each individual installation. But, if you get 'close',
that's probably as good as you can expect, depending on the
circumstances. Having the absolute 'best' antenna in the world is
certainly nice! It isn't absolutely necessary. Everyone goes through
this same sort of thingy. The results differ, but so do the
circumstances.
Good luck.
- 'Doc