If they were tempering, then they were attempting, with at least some degree of success, 12-TET.

That's rubbish, I'm afraid, Josepp. There are many temperaments and all but one are most definitely not 12-TET, nor was 12-TET seen as an ideal in the past. It has been known about much longer than it has been used. Bach called his set of preludes and fugues "The Well Tempered Clavier" for a reason.

A common temperament (more correctly set of temperaments) in Bach's time were the various meantone temperaments. These basically aimed to produce pure major thirds at the expense of slightly out of tune fifths. Meantone worked well in closely related keys but the further you got from the "home key" the instrument was tuned to the more discordant some chords became until you ended up with a "wolf tone" (i.e. a horrible discord) somewhere round the other side of the circle. Well temperaments aimed to remove the wolf tones so that it was possible to play in any key. At the same time well temperaments aimed to preserve the varying character of different keys, (something that 12-TET fails to do) which was considered desirable in Bach's time. There has been controversy over exactly what temperament Bach used, but there is general agreement these days that it was not 12-TET.

That said, the circle of fifths is still a very useful tool for thinking about relationships between different keys and although traditional instrumental music uses very few keys, singers will sing in whatever key best suits their voice so a wide variety of keys are used in practice in traditional music.

Bach Prelude No. 1 in Cm played using three different temperaments. It's worth listening as the differences can be clearly heard, especially with the Lehmann which at first doesn't seem to sound "quite right" to our ears as we are accustomed to 12-TET.