Hello again,
I've been out the last week and left all the gadgets with access to internet at home and that's why I didn't get the chance to reply.
Regarding Canon:
1. Forget about 50D. It's very old with a very old sensor and poor af. If you had a 600mmf4L lens for it, it would be a little different, up to a point. But paring a 50D with a mediocre lens like Sigma 150-600mm lens won't help.
2. Sigma/Tamron 150-600mm vs Canon 100-400mm. Tamron G2 lens is quite good. Is not that sharp or fast focusing as Canon lens at the maximum focal lenght, but you will get extra 200mm by going to Tamron.
3. 5D Mark IV will be very good for wildlife (action or stationary subjects).
4. 7D Mark II vs. 80D, I would choose 80D because it's better at high ISO and the af is not far behind either. It also has better dynamic range.
5. Canon 100-400mmL lens is very good. It's fast to focus, sharp wide open and light. With 5D Mark IV you will have tracking af even with 1.4x TC added on the lens. But you will shoot at f8 if you add TC and at f8 the Tamron 150-600mm G2 is quite good also at 600mm. I would pick Tamron G2 if the reach is important. You can shoot at f6.3 and 600mm with the Tamron lens and you will keep also the ISO lower than when you shoot with Canon and 1.4x TC.
Regarding Pentax:
1. If you don't plan to shoot action most of the time, you won't have problems with K1's af. K1's af is ok, not great for action (some may argue with me on this, but K1 wasn't designed for speed) but with proper tehnique you can make it work for action too. Given the fact that you said you're not into BIF or fast action, the only thing that may drive you crazy is the slow reading files to memory cards.
2. Pentax K3/K3 II... the "problem" I have with K3 is the ISO. Depending on the available light and the quality of it, after ISO 800 the K3 files don't look that good. And if you shoot with Pentax 150-450mm at 400mm f5.6 (or f8 and 630mm if you add TC) you will need fast shutter speeds and this may lead to high ISO. On the other hand you will have 8fps on K3 and half on K1. You will also have faster reading files on memory cards with K3.
3. Pentax 150-450mm lens with 1.4x TC. I haven't shoot with this combo on Pentax and I don't know how is the af and if you'll have af on K1 and K3. Someone said above that af is ok when a 1.4x TC is added. I'm a little skeptical (without trying to offend anyone) when comes to af because both K3 and K1 have af points sensitive to f5.6, not to f8. The cameras that have f8 sensitive af points are usually the more expensive ones (D500, 80D, 5D Mark IV, 1Dx II, D5, etc.) and with those cameras you will have af and tracking at f8 when a TC is added on the lens. I will try and see if I can test a K1 with 150-450mm lens and 1.4x TC and I will get back with my impressions.
You probably won't go wrong with either K1 or 5D Mark IV. To mee, ISO is important because you don't know how the light will be on location and the difference in files will be visible between K3 and K1 or 5D Mark IV at ISO above 400-800.
I hope these informations helps.