Does the Forum think it worthwhile trying to find an accurate load in a 16.5" Bushmaster 1:9 H-Bar 5.56 with thousands of rounds through it?

Yes

you never know without trying.... good barrels can last 10K rounds and hold 1-2moa fwiw...

Others die in 2500 rounds.

I'd start with a 69 Matchking and some Varget.

Sell it to someone who can't shoot but doesn't know it and he will make claims of his rifle prowess. Making something marginal into something great is usually futile. These guns are still desirable and you might as well sell it, start with a good gun and wear it out yourself, rather than duct tape some project gun back into useable shape...

Yes, very worthwhile.
I have a Bushy with 16", 1/9 twist, 5.56 chamber.
This gun groups 55gr. ball ammo 1-1/2" to 2-1/2" at 100 yds.

Shot some load workups for it yesterday with 55gr. Sierra HPBT (bullet #1390) at 100 yards. Ramshot X-Terminator powder.

And on the flip side I had a bushy barrel with 1500 rounds in it given to me.

Shot some proven match ammo in it, stuff that always shoots under MOA..

Would hold around 2moa..

You won't know, until you try.