Found the following article on the Canon close up lenses from the EOS magazine. Was interesting to me, although all the examples are based on using the 500D and 250D on Canon lenses.
https://www.eos-magazine.com/EOS Collection/shop/Resources/A383 Close focus (CU).pdf
The article notes that the lenses reduce the minimum focus distance of the lens, allowing you to get closer to the subject for more magnification. I might like that better with a frog than a poisonous snake.
When I was in Costa Rica a few years ago, I used the 70-180 mm Micro Nikkor lens on a D500. Worked well for frogs. Did not try snakes. I also had the 300 mm PF along, which has a nice minimum focus distance.
The reproduction ratio for the Z 100-400 is 0.38. If you add the Z 1.4x TC, it becomes 0.53. That might be enough magnification for many purposes, while retaining a nice working distance, although you lose a stop of aperture. The EOS article suggested that you do not lose light working with a close up lens.