I see. Feel free to make your own build with configuration files included. Don't forget to adjust uuids.
This cannot be implemented universally. You would need the uuid of the drive the user is adding in advance. What would work only if you format blindly every attached drive with a specific uuid which cannot be random. But uuids have to be random to avoid conflicts.
The "bottleneck" is either the hardware (cpu, ssd or hdd, i/o interfaces) and/or the processes/services (waiting for other processes to finish; other dependent process is starting before/after) launching on startup.
You did already what can be done with changing priority. As this thread is older things could have changed. You have to look into the scripts in /lib/preinit to see what is going on.
But if I look at the 71_myload_raid script I see it is relaying on initramfs/initrd. OpenWrt is not using initramfs/initrd by default. It might be the writer is using a x86 based system using initramfs? So the issue is there.
It's just I've seen many people who try to achieve their goal using unsuitable tools.
They spend days and weeks and mostly fail.
But, even if they eventually succeed, the wasted time is rarely worth the effort.
And typically, they could get the same result working out of the box with the right tools.
openwrt has supported initrd for a loooooooong time... as has the buildroot and compiling in kernel modules needed at boottime...
only an absence of skills, patience and a willingness to learn stand between you and your goal.... improve your skills and knowledge, then you can look at improving those of others... and / or passing an informed judgement on what is and is not available...