I've only dipped a little into the add-on market. Bought a Matrix for G1 Prime from Transrepro back in the day, and the G1 Prowl and Tracks heads for RiD Prowl and Sideburn (looking back, 20-y.o. me was kind of a dork! Thankfully, I quickly got out of that stage and reversed that error!)
And a few sticker sets from Reprolabels before they became Toyhax and went all nuts with the upgrade sets.
And a couple Targetmaster partners of various quality.
And, most recently, some replacement faceplates for TR titanmasters that I still haven't gotten around to using.
Ultimately, although there are custom projects I still want to do when I have time and space, I've come to the realization that, for my collection, I could spend a lot of time and money making my Transformers "perfect" (which would still not be truly perfect, how many different versions of various characters have I bought over the years because they were slightly "better"?), or I could enjoy them for what they are, and maybe pass that enjoyment on to the next generation. The add-on kits have their place, and I can absolutely see why some fans would want them, but they are definitely not for everyone.