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I was not going to make this change by myself, and I was content to leave the status quo unless a pull request providing an implementation was provided and the consensus on @rouault's implementation was generally well received by the mailing list.
After having started tackling this issue in #1234 I've been wondering if it would be better handle this in a slightly different way. I still want to switch the default tmerc to the etmerc algorithm. But instead of creating a stmerc that is a copy of the current tmerc I would add a flag to tmerc that toggles which algorithm is used. Perhaps +fast or +approx. This is similar to helmert where it is possible to toggle the exact version of the algorithm by adding +exact.
The main advantages to this approach is that no further (dummy) operations are added and possibly easier to document (we can stick to just one Transverse Mercator doc page).
@rouault will this make any difference in the ISO19111 code? Currently etmerc can be toggled on and off with USE_ETMERC=NO, at least my alternative approach suggested here is closer to that in the projstring but I can't tell if it could also simplify things in the C++ code.