Steve:
If you have poly on the wood now, you should stick to poly. Nothing sticks to polyurethane like the polyurethane from the same manufacturer.
I don't know why you thinned the poly, but the Minwax stuff is so forgiving it simply isn't needed. In fact, thinning that product can be counterproductive as it cuts down the layout time which diminishes application marks such as brush strokes, or light/heavy gun passes.
You should be aware as well that polyurethane has diminishing returns on multiple coats. Eventually, you will probably see witness lines as well as adhesion problems between the coats (depending on how they were applied).
Polyurethanes will not build a monolithic finish, but will remain in layers as their resins and solvents do not dissolve in to the previous coats. There is a "second coat" window as well, and if you don't observe it, you will be putting down new finish coats that adhere, but not as good as possible if they were put on in a timely fashion.
Again, Minwax poly (solvent based) is bullet proof. Follow the instruction on the can and don't screw around making the finish into something it isn't. It's a plastic resin coating.
If you want to build your coats and goof with a lot of variations on finish, then switch to lacquer. You can build the coats, thin this way or that, sand, polish, and all manner of variation on that theme. But lacquer should go on by itself, no primer, and not over another finish. It will bite into stain just fine, but allow the stain (if oil based) a week or so to cure out before applying your first coat. The stain must be completely dry to the touch before application of any finish over it, but lacquer is particularly unforgiving of the linseed oils and petroleum products in oil based stains.
If you want to make your glossy lacquer finish a satin finish, use the finest scotch brite pad on a block to knock down the shine. Don't use steel wool as it will leave its metal fibers behind and it is much harder to get a consistent satin finish. Also, since the padding nature of steel wool does not provide and 100% consistent, even cutting surface you will find it hard to keep from having inconsistencies in the reflectivity.
To get the best satin finish, DO NOT knock the shine off the lacquer until 21 days have passed from your last coat. It will be its hardest at that time, totally cured out. This will make it much easier to control the appearance, and will keep the finish from "pilling".
You don't need lubricants to do this. No mineral oil, soap, hair oil, motor oil, or some special grit lube from England claiming to be the long lost secret of the ancient craftsmen. The lacquer will scratch easily; brush, and vacuum as needed while working. When you are finished creating your satin finish, take a barely damp rag for final cleaning, and a toothbrush to get the crap out of the nooks.
As far as favorite polys go, I have had good luck with almost all of them. In abrasion/adhesion tests, Minwax consistently performs just as well as the more sexy and harder to find brands. If you like the way it applies and you are happy with your surfaces after you are finished, stick with it.
As far as lacquers go, the stuff I really like is about $65 a gallon unless I buy a case. So when I don't need a case, I tend to move towards the Old Masters lacquer. It has a high solid content and seems to perform quite well in almost all conditions.
Then there is Deft. One of the oldest finishes around, it is an oldie but goody. Often overlooked by those in search of something much more sophisticated sounding and not so pedestrian, Deft is a very solid performer that is a very good finish which is easily repaired if need be. I have had great luck shooting and padding Deft, but brushing it was a disaster for me.
Next, don't buy crappy lacquer thinner. Stay away from the junk they sell at Home Depot, Lowe's, Ace, or any of those places. The stuff they sell is pretty much gun wash and works well for that. Go to a real paint store and some branded lacquer thinner that is the more expensive, virgin stuff. They sell that at Sherwin Williams, Benjamin Moore, etc.
Don't make the mistake of thinking that buying an expensive finish will make your finishes better. But a couple of cans and spray out some different mixes to see what your fun likes. Practice with whatever finish you buy to make sure you aren't practicing on your projects.
Good luck!
Robert