Tor introduced native DDoS protection for onion services using a transparent Proof-of-Work defense over a year ago: https://blog.torproject.org/introducing-proof-of-work-defense-for-onion-services/
You should strongly consider making the switch and turning on this feature. I was very surprised to find that this instance was being Cloudflare blocked. Maybe we could help with funding if needed.
To be honest, I didn't verify the signature when I installed it. The download is over TLS (HTTPS) so you know you have a secure connection with the F-Droid server during the download. But because of the tiny chance the F-Droid website was hacked at the time you download it, you should verify the signature.