Is there anybody out there with a non-financial background who used Schweser and only Schweser for Levels 2 and 3 and got through?
Yes I did (though I also used other tools like secret sauce, question bank, and CFAI exams/mocks)
I’ve used Schweser Notes only for all three levels and passed all exams on the first try, with a wide grade margin. I had science/engineering education up to Master’s level, but no accounting/business/finance classes prior to taking CFA. I do work in finance, but I write quant programs, so my expertise is quite narrow. If you have good math/analytical skills and background, I would recommend using Schweser (notes only, the rest is redundant). For good retention, do all of the practice questions right after you finish each section. You’ll probably be done in <120-150 hours and will be less likely to drop out due to boredom and fatigue from wading through useless fluff and ramblings in the curriculum. On a given test, there were in my experience only 1-2 questions you’ll miss or guess because of Schweser, usually due to alternate terminology that they trip you up with. (Except Ethics - see below). The biggest challenge to me was accounting, as sometimes the conventions did not appear logical, but you are up to speed on that after level I. One exception may be the Ethics section. You may want to read through all the case studies and questions in the CFA curriculum. The exam writers really enjoy testing the special corner cases and splitting hairs, some of which Schweser may omit. Ethics is the only section that I could not get >70 with Schweser alone. Past L3 written questions (freely available from CFA) are extremely important for L3. Also, I would recommend doing CFA practice exams, to get a good feel for the difficulty level. Schweser exams tend to have more math and accounting work compared to the real thing, though they are good practice.
I used only CFAI materials and really enjoyed reading them. If you have spare time, I’d recommend not looking for shortcuts.