Recommend the supplements and gear you actually use, earn on what your clients buy, and keep it on the same page as everything else you sell.
Coaches get asked which protein, which shoes, which creatine, every week. Put the answer on your page with your affiliate link on it and the recommendation you were giving away starts paying.
Assign a pick to a client and it appears in their app with your note on why. A recommendation attached to their programme gets bought; one buried in a chat from March does not.
Sleep, recovery, training, kitchen. Group your picks so a client reads a shelf rather than a list, and so the thing you actually want them to buy is not the eleventh item.
Affiliate picks and the things you made yourself live on the same public page, so one link covers everything you would ever point a client at.
Yours. You bring the link, you keep what it earns.
No. That is between you and whoever runs the programme.
Yes, with a note on why, and it appears in their app.
Yes, by whatever makes sense to explain: sleep, recovery, training, kitchen.