My children's clarinet teacher seems to be suggesting the 'landmark exam' route.
DS has never taken an exam, as he used to be on the in-school 'stages' scheme moderated by the LEA teachers, and since that petered out with the removal of all LEA peris has just continued to play.
His great love is jazz, and he has moved up through e.g. county jazz groups, into the school orchestra, been selected for concerts etc on the basis of his playing rather than any piece of paper.
However, his teacher has suggested that he should do Grade 5, probably at the end of this year. He has, I think, learned all the pieces - in amongst a constant diet of new pieces, learning to improvise properly etc - and returns to them periodically once he refines aspects of his playing on other pieces (so if, e.g. fast runs are proving tricky, he gets some other pieces with lots of fast runs, focuses a lot on those, then returns to an 'exam' piece with improved fast runs IYSWIM).
I don't anticipate him doing any others for a while unless he wants to. Grade 8 would obviously be a final aim, though what may happen instead is that he takes up sax as a parallel intrument and decides which to take forward to higher grades.
DD - currently around Grade 3-4 standard - loves the absence of exams (her main interest is dance and her progress through that is wholly through exam) and may never take a music exam unless she actively chooses to do so.