The problem, AIUI, with TC's is that you're putting more glass in front of quality macro lenses, which probably means that what you gain from having the TC you may well lose in quality. Whereas, with tubes, you lose nothing in quality, but do lose light.

I don't own any TC's as yet. I have a set of extension tubes and have put them on a macro lens and the DoF becomes vanishingly small. I've had fun with them on telephoto zoom lenses, though not tried them on prime lenses which is meant to be good.

I have the Sigma 180mm and it's very nice - but does break the bank

I'd recommend looking at a second hand Pentax 100mm Macro lens or a second have Sigma 105mm Macro if you can find either. The latter you might be able to pick up for <£200, the former depends on what you get hold of. There are manual versions which you can sometimes find for a good price.

I also have the Sigma 50mm Macro which is lovely, but with that you need to be around 5cm from the subject to get 1:1.

A cheaper alternative would be to start with a good lens and use extension tubes.

I've converted from using natural light to flash (with a soft diffuser, aka a plastic milk carton) on a flashgun as it helps so much with DoF and the poor light we've had in the UK this summer.

It's only from experience on one outing a while back, not from any technical evidence:

I don't have the before and after pics anymore as I was disgusted with myself and deleted them (I don't do that anymore).

I went to the local lake to photograph water birds using a Sigma 175-500mm, mostly at/near the long end of the zoom and using AF.

I spent a couple of hours taking shots with the camera on a nice solid tripod. I then noticed that I'd left SR on so thought little of it and switched it off. Continued taking pics, all looked fine on the LCD.

Got home and transferred pics over to PC and found that nearly all of the pics with SR were just ever so slightly soft focused on the central subject, which pretty much ruined the shots for me. For the shots with SR off, the pics were of my own personal standard (!).

My conclusion was that having SR on either caused or contributed to the duffers. It could have been other factors (lighting conditions changing comes to mind), but it seemed to make sense from the pics I got.

I've definitely seen issues with SR left on when on a tripod.

The main issue with SR is if you leave it on when the camera is on a tripod as it can cause issues - which I've seen with a long lens. Other than that, it ought not be a problem leaving it on unless you have a stunningly steady grip

Yes, mine has the Macro switch that limits the zoom to 200-300mm. I just checked the box and it does have DG on it along with what you've seen. So, the one you have seen is likely to be the previous model, though it's important that it is still an APO version.

There was a short discussion I started when I recently got this lens:
https://www.pentaxuser.com/forum/viewtopic.php?t=5250

I like the site, but I'm afraid that as soon as I hear music coming from any site I close it. I'd suggest disabling the music by default and leaving it as an option for the visitor. Imagine people browsing from a workplace and having music suddenly blaring out of their PC speakers around the office.

Lovely macro shots, trying to get pollen is fun

I have the 50mm sigma macro lens too and it's great fun to use. I like the red rose pic - as you say just slightly off on the point of focus, but I like the lighting and composition.