Is it possible to reboot server from /dev/sdb while RAID 1 rebuilds /dev/sda?

I have a dedicated server with a Raid I with /dev/sda and /dev/sdb in which /dev/sda started to fail so I ordered its replacement. After the intervention the server went into rescue mode and I was not given any info from support so I've been trying to get it back up.

I realised even though they replaced the defective hard drive, they didn't bother to copy over the partitions from /dev/sdb and add the new hard drive to the mdadm. So after doing this myself, I see on /proc/mdstat that the recovery is now underway.

root@rescue:/mnt/etc# cat /proc/mdstat
Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5] [raid4] [multipath] [faulty]
md2 : active raid1 sda2[0] sdb2[1]
523200 blocks [2/2] [UU]

md3 : active raid1 sda3[0] sdb3[1]
20478912 blocks [2/2] [UU]

md4 : active raid1 sdb4[1] sda4[2]
3884961728 blocks [2/1] [_U]
[====>................] recovery = 22.4% (872776320/3884961728) finish=254.3min speed=197355K/sec
bitmap: 1/29 pages [4KB], 65536KB chunk

unused devices:
I searched on my backups for the appropriate root partition on fstab:

#
/dev/md3 / ext4 errors=remount-ro,usrjquota=quota.user,jqfmt=vfsv0 0 1
/dev/md2 /boot ext4 errors=remount-ro 0 1
/dev/md4 /home ext4 defaults,usrquota 1 2
/dev/sda5 swap swap defaults 0 0
/dev/sdb5 swap swap defaults 0 0
proc /proc proc defaults 0 0
sysfs /sys sysfs defaults 0 0
/dev/sda1 /boot/efi vfat defaults 0 0
tmpfs /dev/shm tmpfs defaults 0 0
devpts /dev/pts devpts defaults 0 0
/usr/tmpDSK /tmp ext3 defaults,noauto 0 0
And I made sure it was intact by mounting it from rescue mode.

Then I tried netboot specifying /dev/md3 as the root partition. However when I try to do this the server goes immediately into emergency mode explaining there are some issues with /dev/sda (I imagine because it's still being rebuilt).

FAT-fs (sda1): bogus number of reserved sectors
FAT-fs (sda1): Can't find a valid FAT filesystem
I can't pass the login prompt as my root password seems to be unrecognised (using KVM), so I don't really know what the results of journalctl -xb show, but I imagine the checksum for that hard drive isn't adding up.

The question is, is there any way to restart the server using the raid but prioritising /dev/sdb while the RAID is being rebuilt? I know that every reboot makes the mdadm repair process go back to 0% so I'm now trying to be completely sure that if I try something else it'll work.