I second this. Make sure you have the Helminth segment from Deimos and then start replacing the frames you enjoy with their Prime counterparts.
There are a variety of mods that are commonly used in the higher end steel path builds. Brief Respite, Rolling Guard, Adaptation to name a few that come up near universally.
Feed your old non-prime frames to the Helminth as you acquire the prime versions.
Get some solid multi purpose frames like Ivara (great at Spy, Animal Capture, Fishing, Mining), Mesa (great for hunting and killing Lich and Sisters of Parvos), Nekros/Khora (your resource/mod farming frames), Revenant (generally nigh unkillable)