Pro tip: Just cut the stringers right the first time instead of messing with shims forever
Built a small set of steps off my back deck last month, and I figured I would just cut the stringers a bit long and shim the treads level. Big mistake. I spent 3 hours on a Saturday cutting cedar shims and stacking them, and the steps still wobbled when my kid ran down them. Finally pulled the whole thing apart and re-cut both stringers to the exact rise, which took 40 minutes with a speed square and a circ saw. Steps have been dead solid for 6 weeks now, no shims anywhere. Why do we all try to cheat the math on the first cut when the math is the easy part?
Have you ever noticed that the guys who shim stuff are usually the same ones who never check if their deck frame is actually level in the first place? I ask because I did almost the same thing you did, but my problem started way before I cut the stringers. My rim joist was out of level by almost half an inch over six feet, so even when I cut the stringers perfect, the whole stair wanted to lean. Shims felt like the fix but they were really just hiding a frame problem. A lot of us skip the step where we check the thing the stairs attach to, and then we blame the math on the stringers. Once I shimmed the rim joist solid and re-cut, everything lined up like it should. Check the frame before the stringers and half of these headaches go away.