Hi Kate,
I think the article you have found there explains things really well, I am not sure if I could add much more. Are these the positions you are finding uncomfy?

I know that slouching is not recomended and sitting up straight is better- but i'm not finding the 'correct' positions comfy. Is this normal, and i just have to suck it up, in order to give bub the best chance of optimal positioning?
Slouching is definitely not good but there are different versions of sitting straight. The correct positions should theoretically be comfortable as they are the way the body is designed to be. Sometimes when we have had bad posture for a long time it can cause chronically shortened or lengthened muscles (therefore weak) which can make sitting or standing in the correct positions tiring or just plain straining. It might be a good idea to visit a physical health practitioner that you know such as an Osteopath, Chiropractor, Myotherapist or a Physiotherapist. They should be able to check your posture, advise you on your pelvic position in different postures and maybe correct balance and treat muscles and ligaments which are making holding the OFP postures uncomfortable.

The other reason a 'correct' position might be uncomfortable is because it is not really correct. A lot of times when people try to stand up 'straight' they over compensate and stick out their chest to elevate from a slouch position. This is not a position the body can maintain comfortably as it is not natural. In fact the correct posture is generally somewhere in the middle of a slouch and standing to attention. Again, a practitioner should be able to help, but the best clue is that the position should be comfortable. Try rocking your hips back and forth to extremes to get a sense of where it is on your body and then try to stop in the middle. This should be the natural position. Do the same with exaggerated moves with shoulders. Shoulders forward/back a number of times, stopping in the natural middle. Again with shoulders up and down. You can apply this to any part of the body as a sort of reset for posture.

When I was working on OFP I was vigilant about never sitting on the couch in my last 10 weeks of pregnancy. I used a dining chair or sat on the ground. If I had to sit on the couch I would perch on the edge of it rather than sitting right on the cushions incase I fell back into bad habits and slouched. I also invested in a kneeling chair - second hand on ebay - and used that a lot.

Try to avoid driving if possible (long distances) as car seats are appalling for posture. It is worth using a towel as a wedge or a well positioned bolster if you do need to be in a car.

FWIW I never had kicks in the ribs with either of mine and they were both born anterior. I did notice the hands and feet to the front though. I suspect that if you have a smaller torso or a longer baby you may be more likely to get the rib kicks due to the space issue.

Also, now is a good time to think about scrubbing the floor by hand