hello
If by side-loading you mean installing from a ‘non-trusted’ or ‘unofficial’ source (apps other than google play store), then yes, that is the only option. however, this is not much less convient or secure.
There are a few ways to install apps (these should work with any Android phone, not just Grapheneos):
Any non-paid apps on google play can be installed using Aurora Store. supposedly you can use it without even logging in to google account (but i havent gotten that to work). Eg. Facebook
Most FOSS apps will be in the F-Droid repository, and can be installed with the F-Droid app, or other similar apps such as Droidify. Eg. K-9 Mail
If an app is not on the main F-Droid repo, alternative repositories can be added to your F-Droid client.
APKs (app packages) can be installed from the web. this is how F-Droid is usually installed for the first time. Eg. Signal is not available on F-Droid (for reasons)
Basically, if you need a proprietery app like Facebook, use Aurora; if you need an open source app, look for it on F-Droid or download the APK. There are of course pros and cons of each method in terms of security and convenience.
the android which most phones use is android+gapps (google apps), which is a closed source fork/distro of android made by google. base android and grapheneos (based on android 13) are open source. grapheneos does not have google play services, so most google apps will not work at all, but play services can be installed (although i dont recommend).
personally, my bank app works out of the box, even without google play services. i would recommend not banking on your phone unless you absolutely need to, like mandatory 2fa (only using the mobile app)