Great list! It’s nice to see a fellow Slowpoke evo line collector! Skimming through, I only noticed two missing variants (that I’m aware of). The Domino’s pizza 028/L-P Slowpoke had two print runs; the “たね” text in the first attack is formatted differently depending on whether it was released with the Slowbro or Slowking promo.
The second variant is much more minor (you may not even consider it a proper “variation”), and one I’ve had a good deal of trouble identifying from online listings. Vending Series 3 also had two print runs: a dark print and a light print. Due to differences in scan saturation and other similar factors, it can be pretty difficult to differentiate between the two online, but the difference should be pretty noticeable side-by-side, more so than the standard deviations in print color of any set.
Hope this helps!
(I learned of the Vending Series 3 variants from Quuador’s collection thread here, which provides a great example, and the Domino’s variants are noted on the card’s Bulbapedia page).