Any advice or insight is appreciated!
Have you checked the nrmp charting outcomes for your specific score in PM&R? (I think the lowest was right at 500 last year), but keep in mind this is just one factor. If you do absolutely nothing to compensate, then I would say you definitely have reason to worry.
Personally I think your chances are what you make them. Some programs do care excessively about the paper portion of your application, but others are looking for more than just numbers. Scores are important, but you can't change it now, so my advice is probably just stuff you already know:
1. Do better on the next exam.
2. Try to do PM&R rotation(s) / research and build a relationship at a non top-tier program.
3. Get LORs from physiatrists.
4. Buff up the rest of your CV as much as possible.
5. Be prepared to discuss your score, without any hesitation, whether in interviews or anywhere else. It's only a negative if you allow it to be. Don't even bother giving an excuse unless it's genuinely a unique/ life-altering type of issue, (eg major illness/ death in family- yes; result of being a human being- no), describe how seriously it affected you, mention multiple things that you learned from it, the changes that you made, what you did to ensure never repeating that mistake, and how you weren't going to let anything deter you from pursuing your ultimate goal of becoming a physician/ physiatrist.
6. Reassure yourself- you are still more competitive than say someone who failed it, or failed a year in school, IMGs, etc.
7. Get advice from multiple sources.
I hope this helps and good luck!