Too rare and it is impossible to build a collector base unless as part of a broader collection.
As a case study, it is only in the last two or three years that thrymsas have become collectable. This is due to the still small, but larger numbers than before the metal detectorists meaning that the dozen varieties can now be assembled, whereas previously you would have been lucky to acquire more than one or two. It is for the same reason that many avoid proofs and patterns which are frequently only known in single figure quantities for any given variety. As a consequence, the denomination collector will likely acquire one or two examples only of the commoner types, if at all.
Forget the ESC rarity values. There are R6s and R7s with double digit populations. Conversely there are Rs and R2s which never appear. Do your homework. The rarities are likely to be highlighted in auction catalogues, so all you have to do is work out which ones appear, how often, and how many of these are the same pieces reappearing.