Sigma 150mm is a FANTASTIC LENS - the older version is still a very good buy and very cheap on the used market now.
That aside a new macro lens won't give you any more magnification than you're getting now. 60mm, 35mm, 150mm, 200mm - because magnification is a ratio of the reflected image size to the actual subject size the 1:1 magnification that all the current prime macro lenses on the market get to gives you the same frame coverage no matter the focal length.
What changes is the working distance (longer macro lenses give you 1:1 from further away) and also the background blurring with longer lenses giving an increased amount of background separation and thus more blurring.
If you want more magnification you've already identified the 3 key methods to use - extension tubes, teleconverters and close up lens attachments. Reversing is also a fourth option though, as you rightly say, you'll need a new lens for that as well. The only way that you can increase the magnification without losing focusing distance is to use teleconverters, you might get luck with the Kenko pro brand teleconverters (they have the smallest protruding front element and thus fit to the most possible lenses) however do check to see if its possible before buying. Another option is to use a teleconverter with an extension tube between it and the lens - this will still remove infinity focusing on the lens and reduce the min focusing distance, but it will still let you mount the telconverter to get its bonus to the magnification.
The close up lens attachments are also a valid option, again they cut your focusing distance, but they work well and have the bonus that they are fast to attach and remove. A Raynox DCR 250 is a great and affordable start in this regard.
As for working distance, don't get too scared, the only lens on the market that currently does more than 1:1 magnification is only 65mm in focal length and drops down to around a 40mm at its 5:1 setting - and many photographers use that lens all the time. A challenge yes, but sometimes those reduced working distances make other things easier (you can brace yourself on the surface the subject is upon more easily and thus reduce shake, which becomes even more extreme as the magnification increases_/