You've misinterpreted the level ratings in WCAG: they're not "priorities", they're different levels of conformance. Most organizations aim for at least A-level conformance; few go all the way to AAA-level conformance because, as you point out, that level of accessibility is challenging to meet. In fact:
"It is not recommended that Level AAA conformance be required as a general policy for entire sites because it is not possible to satisfy all Level AAA Success Criteria for some content.") http://w3.org/TR/UNDERSTANDING-WCAG20/conformance.html
To consider your product accessible, it must meet level A, it should meet level AA, and it may meet level AAA.
You may also wish to review the VPAT template at https://accessibility.oit.ncsu.edu/training/accessibility-handbook/vpat.html, which is essentially a self-assessment checklist of "does this product meet WCAG?" in standardized form.
(I'm coming back late to edit this answer to echo an important point made in other answers: the checklist is only the first step; it's important to actually test the design for accessibility and make sure it's truly usable by all. It's quite possible to build a product which technically meets all the guidelines, but is still unnecessarily difficult or annoying to use.)