I have never tried to do what you are doing but 700MB is unreasonably huge for an initramfs. Are you sure there isn’t something wrong there? That would mean that you could only fit a single initramfs in a 1GB /boot partition.
Also, initrds are install specific so to take one from another place and use it would require an identical configuration in both places.
tbh I dont why initramfs is so huge…probably because I build initramfs in arch linux, yeah I compiled the kernel and I used fedora kernel config as base config
a lot of modules are missing when I try to boot fedora with initramfs which are built in docker container(fedora), whole networking is not working and usb etc. is not working
for example when I build initramfs in fedora(qemu) I didn’t get any warning/error message, the size of initramfs is around 70MB, qemu boot works perfectly (the network,usb etc. module are found)