I live in fly fishing country, so I don't use my spinning setup more than once or twice a year these days.

That said, the last few trips I've used both of these setups:
6'6" St Croix triumph (2 piece), Medium Fast, Plueger President 6730 (size 30?)
6' Medium Heavy Berkely Pulse (1 piece), Shimano 4000 (a nicer one).

I'm looking for something in between these setups, a little longer and stiffer than the st Croix, but not that wears me out like the Berkely

I target northern, Smallmouth, walleye, and hopefully a laker. Most of the time the St Croix does fine but is slightly tip heavy and can definitely stress on a big northern. I also like being to shift between sample and heavy lures at will while fishing too. The last few trips I've also learned light tackle works just fine for bass and walleye.

What do you guys suggest? A 7' medium, Pflueger size 40? Will this combo be balanced?

My advice would be to get a unique handle.

I would suggest two different outfits. A 6 1/2 or 7 foot medium light action rod, with a Shimano, 2500 reel , spooled with 6 or 8 pound test for Walleyes and Bass. I would take a medium heavy, 7 foot rod, with a 4000 series Shimano reel, with 12 pound for Lakers and Northern…

I just fished with a guy who was using Fenwick rods, one of them was a 7-footer. Although it's medium-fast, this might work for you. There are also options to get it with a stiffer action.

Argo1: "...
I'm looking for something in between these setups, a little longer and stiffer than the st Croix, but not that wears me out like the Berkely
"

I wonder how a brand of fishing rod "wears you out"?

bobbernumber3: "

Argo1: "...
I'm looking for something in between these setups, a little longer and stiffer than the st Croix, but not that wears me out like the Berkely
"

I wonder how a brand of fishing rod "wears you out"?"

I have a wrist injury. My Berkely rod has a weird divet where your palm goes. Also it is short stiff and when using heavy lures it just strains my wrist. Not entirely the rod's fault, but also why wonder if I can get away with a longer rod and not medium heavy.

Argo1: "

bobbernumber3: "

Argo1: "...
I'm looking for something in between these setups, a little longer and stiffer than the st Croix, but not that wears me out like the Berkely
"

I wonder how a brand of fishing rod "wears you out"?"

I have a wrist injury. My Berkely rod has a weird divet where your palm goes. Also it is short stiff and when using heavy lures it just strains my wrist. Not entirely the rod's fault, but also why wonder if I can get away with a longer rod and not medium heavy."

I can echo Argo's comment... sounds like you need a unique handle in two ways!

Argo1: "
I target northern, Smallmouth, walleye, and hopefully a laker. Most of the time the St Croix does fine but is slightly tip heavy and can definitely stress on a big northern. I also like being to shift between sample and heavy lures at will while fishing too. The last few trips I've also learned light tackle works just fine for bass and walleye.

This is a tough ask for 1 rod. If you insist on only using 1 rod, then I'd say you're already there, with your 6'6 medium/fast, as it's right in the middle of the power spectrum.

However, I agree with walllee above. For walleye and bass (finesse presentations), get a medium light rod. For bigger fish such as pike, go with a medium heavy. These are the 2 rods I take.