r134a from a security perspective it's adviced to not allow it to update that app then from another source (obtanium then in this case), as it would allow generally from that point on updates to this app from any source.
That's not true, any app installed for the first time has its signing key pinned in order to cryptographically ensure that all future updates originate from the same app author. App stores can silently update apps if they were the last source the app was installed or updated from (as listed in the Settings app) and if the updates are signed with the same signing key. If it wasn't the last source then it requires user interaction to update and will prompt you, but once you approve the app to be updated it should succeed. There's no security risk here, the risk is in the initial installation only when the signing key isn't pinned yet.
r134a reinstalling them through obtanium
That's terrible advice and contrary to what's generally recommended. Obtainium is untrustworthy and, in my opinion, worse than getting APKs manually. Again, that's not a problem for updates, but you're telling them to reinstall through it. I recommend against getting apps for the first time or reinstalling them through Obtainium or any other shady source.