Roofing shingle will work, not as good as self leveling compound. Shingles will off gas and has a odor to it. It may make a crinkling sound if, there is deflection in the subfloor frame. A bundle of roofing shingles is not cheap.

If, using self leveling, you would need to glue flooring over areas that are too thick. You would need to wait until the SLC fully cures before laying wood floors over it. Each brand have different curing times. The fastest with 16 hour drying is Henry's 555. You are paying a premium price. Can be costly for large areas.

I would pull the subfloor and sister up the joist to repair the flatness. This would be the fastest and easiest way. You can lay flooring immediatley following completion.

@G & S Floor Service thank you so much for this detailed answer. I was under the impression that self leveling compound cannot be nailed into, so I would glue over the areas with the compound?

In contrast, I would not recommend a cementitious type patch and would recommend using roofing paper, not shingles. We regularly use it to shim under nailed flooring and as long as it's not built up too thick, there should be no issues.

One can use 15lb. or 30lb., though the former may be hard to find because I don't believe it's used for roofing any more.

https://www.flooranddecor.com/installation-supplies-wood-laminate-installation-materials/grip-rite-15-lb.-building-felt-paper-954201960.html?externalVisit=true&gclid=CjwKCAiAqt-dBhBcEiwATw-ggPMMw2rEvHzTlrd4kmLG8ql-27MZcbweSti687_P7UrkSPimdwpDcxoCe7oQAvD_BwE&gclsrc=aw.ds

@Timothy Winzell Thank you so much. I was wondering about roofing paper as I actually have a roll here. What would you consider too thick? If i had to layer several sheets to fill 1/4" gaps would that be ok, in your opinion? I prefer roofing paper over shingles too now that you suggested it.

I have never noticed a smell from roofing paper on a job. Multiple layers can be used. We generally use a smaller piece in the lowest area, covered by progressively larger pieces that overlap the edges of the pieces below to provide a reasonably smooth transition. We have used 15, 30 and even 60 lb. felt. I've had installer use layers of Thermoply for shimming, but one must be careful to feather out the edges.

As long as you stay below 1/2" buildup, using 2" long fasteners, you should be okay.

If, SLC is installed properly, does not flake and is no more than 1/4" thick, nails and staples will shoot through it clean without shattering the SLC. The wooden subfloor will still have a lot of holding power. 2" flooring staples will shoot through the subfloor by 1/4".

Improper installation of SLC will flake and shatter upon impact.