A very interesting topic with good info. (Such as the points about using 90 mm TS/E for macro work).
Your plan to start out with an etension tube sounds like a good way to gain some experience.
My experience is solely with the Canon 100 mm L macro.
I like this optics very much - good build, handles well, great function with many uses.
And I am very pleased with the IQ.
I am still in the learning-phase regarding macro-photo, but find it both fun, challenging and rewarding.
For me the 100mm length seems to be the right choice (on FF with my 6D). Also very useful for portraits btw. If you work with things like nervous insects you should consider also longer FL.
I attach a few samples:
1. Live insect (but not in flight), AF is a must, and IS is very much an advantage here. 1/60s, f/11, ISO 800. Approx 60% crop.
2. Same image, but 100% of cropped resolution
3. Food close-up. 2.5 s, f/18, ISO 400. Manual focus, tripod
4. Food / Spices, 1/13s, f/22, ISO 400. Manual focus, tripod
No 4 Illustrates sharpness well (even at f/22) - notice the thin red thread between cinnamon and peppercorns.
Appreciate comments - and look forward to hear your experience.