My suggestions for you Rollei, are based on establishing sound fundamentals, so you can learn by experience, repetition, and build a strore of experience in film stock used; ISO/ASA/Exposure Index settings and how those relate to light metering; film developing time and temperature, dilution/strength of developer, and agitation methods and intervals used. In a word: forget two-bath development. Forget Ilford developers with Kodak film. Start over, with some basics, and KEEP the ingredients used consistent for at least six months. Again, leave two-bath developing to the experts like timor,and work on doing the basic, critcal things properly, in a systematic way, so you can see and learn what affects what.
ISO 400 B&W film. PICK ONE stock. ONE. If it is a T-grain film, pick a developer that will work with it. If it is a "traditional" film, pick HC-110 Dilution B or D-76 Diluted 1:1 with water; this is enough compensating action if the development is at 68 degrees, agitate 20 seconds at start, then do 10 second agitations on the minute.
ISO as opposed to Exposure index or E.I.: NO, you do NOT want to stick blindly to the film ISO number on the carton, not necessarily. If, the way YOU do light metering, with your camera, and that lens, with your working methods, and your exposures are as you say underexposed, then you MUST lower the E.I. setting!
The title "How I Develop FIlm" is entirely misleading in terms of picture results for the beginner; it is not just about how the film is developed in chemicals, not nearly as much as the entire PROCESS used to get that film ready for the development stage. Choice of film speed and brand and "type"; exposure metering method; Exposure Index set; camera's lens and shutter accuracy/lack thereof; chemical strength; thermometer's accuracy; agitation method and duration; how the negatives will be turned into final images (scanner/condenser enlarger/diffusion enlarger/what paper grade for a normal neg,and so on?).
My feeling, looking at your first works is that you are grossly UNDER-exposing almost all scenes, mostly likely because of the way you are using your metering, but other factors come in too. A camera with a 60/40 in-camera meter with a wide-angle lens can "see" brighter sky enough to inflate the readings; the lens diaphragm might close down a bit too much for the f/stop settings, especially common at smaller apertures!; unless you know how to do close-up readings, many times the in-camera meter will relay wayyyyy to "bright" a light value, and you'll end up, basically, under-exposing the most-critical shadow and lower mid tones, and then the negatives will have no shadow detail; combine that with these delicate, 2-bath developing processes, and your negatives look thin, and poor. Again....you're putting the cart before the horse with this two-bath development and skipping around from 400 to 3200 film, and so on.
Try ONE film, one fairly mild developer (HC-110 Dilution B), at 68 degrees, agitate 20 seconds at start, rap the tank hard, then start agitating gently 10 second every 1 minute, water rinse or stop bath: fixer: wash: photoflo: dry. Chart your development times. Number the rolls, and keep the records. It seems extremely likely to me that your fundamental issue is using too high of an E.I. for your metering methods and your developing. The ISO rating is only a starting point. but you MUST realize that you need to get some shadow exposure, and I can almost guarantee that you are not getting that with the "green dot, overall scene" light metering procedure you mentioned. Lowering the "ISO" is a starting point to getting more shadow exposure.
You would probably do far better metering the palm of your hand, and then over-exposing 1 to 1.5 stops for light-toned objects or 2 to 2.5 stops for the purest white scenes like snow, or UNDER-exposing by 2 full stops for a black dog in sunlight, 3 stops for a black dog in open shade. Your ISO and exposure settings need to be tailored to your working methods. Pick one film, one developer, and stick with it and work out how to get some visible information in the shadows by lowering the E.I. or by metering in a very different manner.