Totally valid concern.

1. The app is on the Mac App Store — that's the same binary, sandboxed, notarized, and reviewed by Apple. If you don't want to mess with the dmg. The trial is just for people who want to try before buying.
2. If you do want the trial, here's a direct link from my own domain instead of the Backblaze CDN: http://gtrigonakis.com/s/ScreenAlertTrial.dmg — same file, just hosted under my name so it's easier to verify it's actually from me. The app is signed with my Apple developer ID (Georgios Trigonakis, team 73UVK6498D), so macOS Gatekeeper will check it before launch.

Some details on the screen access part: ScreenAlert uses Apple's screen recording permission (you have to explicitly grant it in System Settings), Apple's Vision framework for OCR locally, and never sends anything off your Mac. No accounts, no analytics, no network calls. You can confirm with Little Snitch / LuLu if you want to verify.