I’ve never used bcachefs so no help from me there.

/boot/efi is no longer considered an appropriate mount point

It’s not typical, but it should still work. systemd-boot even looks for the ESP there by default among /boot and /efi
Personally I use /efi.

just create a EFI partition and mount it /efi and let the OS put /boot inside the root partition if it needs it., then throw a UKI on /boot/efi/EFI/Linux

If I’m reading this correctly you mounted the ESP to /efi and then put the UKI in /boot/efi anyway. It needs to be on the ESP.
My /etc/mkinitcpio.d/linux.preset looks like this, if it helps. If you were to use /boot/efi for the ESP you would have to change the paths here.

# mkinitcpio preset file for the 'linux' package

#ALL_config="/etc/mkinitcpio.conf"
ALL_kver="/boot/vmlinuz-linux"

PRESETS=('default' 'fallback')

#default_config="/etc/mkinitcpio.conf"
#default_image="/boot/initramfs-linux.img"
default_uki="/efi/EFI/Linux/arch-linux.efi"
#default_options="--splash /usr/share/systemd/bootctl/splash-arch.bmp"

#fallback_config="/etc/mkinitcpio.conf"
#fallback_image="/boot/initramfs-linux-fallback.img"
fallback_uki="/efi/EFI/Linux/arch-linux-fallback.efi"
fallback_options="-S autodetect"

its possible that UKI, systemd-boot, bcachefs, and the /efi mountpoint are not a great mix

I don’t see the point of systemd-boot with UKIs. All it does is chain-load the EFI boot stub in the UKI anyway. I just used efibootmgr to create an entry in the UEFI which boots the UKI directly.