ECC RAM won't make your Pi-hole noticeably more stable.

For a Pi-hole running 24/7, the stability gain from ECC RAM is basically zero. DNS queries are small, simple operations, and a bit flip in RAM would have to hit the exact right data at the exact wrong time to cause a noticeable issue – which is astronomically unlikely.

Plus, most Pi-hole setups run on a Raspberry Pi or similar ARM board. Those don’t support ECC RAM at all. You’d have to build a custom x86 server with ECC support, which is total overkill for a DNS sinkhole.

If your Pi-hole crashes, it’s almost certainly a power supply issue, SD card corruption, or a software bug, not a cosmic ray zapping your RAM.

Spend your money on a good power supply and a healthy SD card instead. Your Pi-hole will be just fine.