Looking to sharpen 5-6 chains at a time off the saw.
What would you recommend?
You said "best" and "off the saw" and "Under $300."
There is only one tool that fits these demands.
Use a spare blade and a file.
Mount the blade it in a vice and use chunks of wood between the bottom of the blade and the chain with a wedge for tension.
If you don't have a vice use large C clamps or fab up a wood mount to get it mounted in a fixed position.
ALL and I mean Every stinking chain grinding machine that I've had an opportunity to lay hands on over the years has been pure trash.
I own one and I hate it. It flexes and bends and is never consistent. It burns the chain because it is inconsistent. To stop it from burning I gotta have a squirt bottle of water in one hand and operate it with the other.
I also bought that gold anodized aluminum jig with the little diamond round sharpening bits for use on the chain as well and it's also trash. Did I mention that they are also so agonizingly slow? I mean dead rotting turtle slow.
I had plans to make one - a really solid one from heavy half-inch thick steel but may not realize them as I have discovered a much faster, better, and easier way - - - but it is done with the chain mounted on the saw or a on a spare bar. A Spare Bar works well.
Watching Bucking Billy Ray Smith do his chains I was inspired to use a file. Such a simple skill, and yet so many of us find it intimidating. It's not at all difficult but one does have to do at least three saw teeth on a chain before one realizes how easy it is. Hence the proliferation of pointless jigs and trash machines and videos by countless idiots extolling the virtues of these worthless gizmoes.
I just hand-file them all now. I don't do square grind (that's for green wood) I use a 7/32 Oregon brand fille and get good gullet and my angle is every as consistent as it would be on a good chain grinder, and I Never Burn the chain with grinder heat.