Lukas I would like my device to switch between LTE and 5G according to my usage, like a Pixel with stock OS does.
That sounds like a useful enhancement and you might file an issue on the GrapheneOS GitHub issue tracker.
But since it is a usability enhancement rather than a security issue or a privacy issue, it might not be a high priority.
Also, while it's easy to describe the feature at a high level it might take a lot of tuning to get something that works well for a lot of people.
Finally, as 5G rolls out to more cell sites, this feature's value might go down over time.
Overall, it may be easier for a large company to develop such a feature than for a small open-source project. Perhaps that large company will choose to contribute the code from their proprietary OS into AOSP. If so, I expect GrapheneOS will inherit the feature.
Please note that I do not speak for the GrapheneOS project.