Extension rings force lenses to operate far from their original design parameters, so they often result in tons of spherical aberration, field curvature, or astigmatism.
Some lenses work okay with them, others do not.
If you want more extreme macro, Laowa has some rather wild lenses, like their 25mm.
5:1? That is wild! I didn't know about that one, but they do also have a 100mm 2:1 for $450, which is surprisingly affordable - less new than I paid for my used 105.
Granted I might still (eventually...) go with a TC-20 E III since it's only 10% more and I can also use it with my birding lens, but absent that constraint, I'd definitely be thinking hard about that Laowa 100mm.
edit: Well, I can use a 2x TC with my birding lens if I don't mind losing autofocus, anyway - the ones fast enough to AF with it all cost as much as a car...
Some lenses work okay with them, others do not.
If you want more extreme macro, Laowa has some rather wild lenses, like their 25mm.