OSM-IV. My school is systems based with normal OSM-I and "abnormal" OSM-II.
My focus OSM-II was to do well in classes and trusted that by studying course material and doing well would create a solid foundation for boards.
I didn't start any "board studying" until last week of April 2019, and took both COMLEX and USMLE at end of June 2019. Everyone is different, this schedule worked for me.
Starting second year I used Boards and Beyond in conjunction with my school's curriculum, I noticed a steady and strong improvement in my test averages (mid 80s to low 90s).
I saved UWorld and Pathoma for dedicated, and went through Boards and Beyond again during dedicated.
I paid for cramfighter and it used it for maybe a week, then I fell behind. Same with First Aid. Bought it, read maybe a chapter and realized it was not a productive use of my time. For me, they were not worth the money.
At a bare mininum, you should purchase UWorld and Pathoma. Boards and Beyond worked well for me, for others it was not a great resoruce. I cannot comment on Amboss, DIT, or other study resources. I am not an Anki user and cannot comment. In my opinion, it works for some, but for others it just fact memorization without any application.
I purchased some (~3?) of the NBME practice exams and took those weekly during dedicated on Saturdays to "simulate" test day. I started at the same time, took breaks at the same time, ate the same snacks, etc. For me, this was a good way to help my test day run as smooth and stress free as possible. Overall, UWSA 1 and 2 were the "most" predictive of my score, essentially hitting their average.
I cannot recommend any of the COMLEX based study resources (COMBANK, COMQUEST?). I bought COMBANK and hardly ever used it. For COMLEX, I used Saverese and relied on my school's OMT course which I personally felt prepared me well enough to pass the OMM portion.
I took Step first and then COMLEX 5 days later. I thought this was adequate spacing.