Try the general finishes gel stain.... It does not penetrate or behave differently over the density variations. I think you be so happy that you would build everything out of fir and stain it.
I use a lot of poplar and maple combined -- neither stain well either.
GEL examples:
cheap pine shutter doors. General Finished Gel, no conditioner.
Pine doors (two 10-lite doors joined into a single 20-lite slider).
GF GEL with no conditioner. Pine Same stain and tone as above, lighting different.
Floating Shelves - same stain (GI GEL) as above this is hard maple with NO conditioner. No splotching, etc.
Just for reference - this is baltic birch (with maple legs not shown) tinted with untinted M.L. Campbell oil stain with tinted M.L. Campbell oil stain over it and then finished with M.L. Campbell pre-cat lacquer. Excuse the gouges, the warden can be hard on things. Her materials science understanding does not include metal to wood contact being problematic.
I do agree- that staining to get good results can be very tough. The GEL stain is not magic, but it can do wonders in some cases. In this case, I use it to match color and tone on vastly different woods, some of which are almost impossible to stain evenly.
I am a bit lost here. You already built the stand. Are you going to rebuild it? Cover it?