Similar thread:
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/21455-macro-help.html
In my opinion, 100ish macro is the best focal length for handholding macro with very comfortable working distance. I handheld my Voigtlander for most of my 1:1 shots of insects. And this lens is much heavier than the Tamron and possibly the other two as well.
It depends on the size of the bug, but I found 1:3 to 1:1 would give you the details of the eyes of a regular fly. 1:1 would give you just enough amount of details for a regular house ant's eyes. Working distance for my Voigtlander 125 is 38cm at 1:1, quite comfortable. Of course, shorter focal would give shorter distance, and I just give you an estimate for you. Technical data can be researched online to know the exact distance at 1:1.
Auto focus is useless for insect work, to be honest. AF hunting is due to the camera body, not the lens itself; however, the Tamron has focus limiter. Thus, you can restrict the range so hunting time would be shorter. Front and back focusing only occur in AF mode, and I don't really know for certain the specific for each lens, but sample variation does occur so a few problems does not apply to the whole product line.
However, if you are really into insect macro, you should buy the Sigma 180mm instead or any other 200mm macro you can get your hands on cause those are the ultimate for insect macro.