Doesn't sound safe at all.
Obtainium
Obtainium is a tool that allows you to get and update Android apps directly from the developer in an automated way.
It supports apps that are provided as .apk by the devs on Github and many other sites like Gitlab, Codeberg and even Signal and Telegram official websites.
This allows the fastest possible updates directly after release and makes additional app stores for many FOSS apps unnecessary.
https://github.com/ImranR98/Obtainium
Discussion
I trust this more than Google spy on everything you do and sell your personal info to anyone and everyone Playstore.
I think its pretty safe. If you install the first apk manualy direcrly from the dev it will have the devs signature. In case Obtainium would tamper with the apk in any way, Android wouldn't install it because signatures differ. What are your concerns?