I've had the OncotypeDX and know a little about it. I don't know why your surgeon didn't automatically save tumor tissue, collected during surgery, as mine did automatically so genetic testing can be done. Unless as you mention the tissue sample would be too small? But the tumor removed from me was 5mm and sufficient for two biopsies and OncotypeDX test. I don't think that being on letrozole would have made the OncotypeDX invalid but could be wrong.

Oncotype has other breast cancer tests that might be helpful so you could call them and ask if there are options still available.

It would seem that you need to find an oncologist who's familiar with all of the genetic testing that might apply, including any that don't require tumor tissue samples. There are several companies testing for different things, including well-known mutations, and some can be identified with blood tests. And new tests being approved frequently. So, if your doctor(s) aren't fully aware of all of them, maybe they can refer you to someone with more knowledge who might be more helpful?

And, just maybe, one of the two facilities that did the biopsies has automatically saved tissue for further testing?

In any case, I'd get a second and maybe third opinion by consulting with at least one other oncologist if this is, as noted, an unusual cancer to deal with. There might be a few different treatment paths to consider and you'll want to be able to choose your best options.