You're weighing speed & energy usage vs cost. With a small array, HDDs power draw difference probably won't amount to much and you may hit the RPi's I/O bottleneck before fully benefiting from SSDs speeds, though latency would be better.
But TL;DR: If your goal is to learn and build something cheap, go HDD. If you hate it, you can upgrade over time.
I'm using Bitwarden, Firefox, and Android and passkeys have been working fine for me.
What am I doing wrong?