Am I wrong in assuming that both OS’s should be sharing the EFI and /boot partitions?
Shared ESP is fine as long as you don’t run out of space. Nothing in /boot should conflict but that’s not guaranteed, although having 2 potential boot partitions means having 2 potential grub configs. I’d make sda3 a ~2GB ext4 boot partition just for Fedora (mounted at /boot), and an sda5 with btrfs with a home subvolume mounted at /home, and a root subvolume mounted at /, then mount sda1 at /boot/efi (this is the default layout iirc, albeit with different partitions, ofc). This might be easier to do in the advanced blivet gui.
And yes, Linux’s boot process is a convoluted, fragile mess and there are currently multiple ongoing discussions on how to improve it.