I own a 1989 2.6 and ''I've been there, done that''
At 90K the car satrted to drink oil. As you suggested, I tried different grades/make of oil (expensive synthetic, mineral, snake and everything else in between. I also tried additives all of which made no significant change. When it consumed 1qt/600mi, I decided to have the valve steam seals replaced. Oil consumption was reduced to 1qt/1,500-1,800mi, which is very good. It stayed there for a while, but soon started to drink oil again At 150K, it was 1qt/1000, which is still very acceptable.
At 175K 1qt/750mi
At 200K 1qt/ 600mi.
At 215K, I had the valve job done.
Since then (now at 235K) the oil level barely move between oil changes
I don't think the problem is related to the type of driving. I run my car pretty hard and had exactly the same problems you have. Oil changes have always been at 3,000mi. This is a built-in weakness of those engines. You can try new valve seals (maybe it will work for a while) but eventually, you,ll have to address the valve guides.