What has worked well for me as an amateur painter is to wipe down with mineral spirits. Then I use Rustoleum's self etching primer (like another, I can't find Duplicolor's self etching primer locally), then I use Rustoleum's 2-in-1 sanding primer which is a primer surfacer, it builds up the primer so that you can sand it smooth and get a smooth finish in the paint. Now, I switch over to Duplicolor's products, I use their Primer Sealer which mentioned earlier seals the primer coats to keep the top coats from mixing with the primer coats affecting the final color and shininess of the paint coats. I then continue with the Duplicolor paints. Pretty much all of the spray primers (self etching primers and primer surfacers) are lacquer based, so I am not too concerned about mixing manufacturers products at that level, despite all the warnings not to do so. I figure lacquer is lacquer and to date it has not been an issue for me yet. What I found to be most important while painting is curing temperature. I tend to paint more in the summer, since I have not had AC in my shop until this past summer, and let the workshop naturally heat up. That works pretty well for getting good results. My workshop easily gets into the high '80's in the summer. If you can get ahold of some inexpensive IR space heaters that should help, maybe put the frame and heater(s) into an insulated box for a few hours so that only a small portion of your workspace is heated into the 80 degree range, the heater should turn on and off to maintain the temperature as needed.
One thing I noticed in your photos is what looks to be fresh welds. Make sure you get rid of all the carbon and slag from those areas because they will affect how well the paint sticks. I hate wire wheels on an angle grinder, but that works about the best on cleaning up welds for painting, IMHO.