I actually just wrote a detailed description of my build here, which is very similar to your idea, except without SSD, and including battery.
However, I myself considered and SSD and ran a few tests; I decided against using it, because of the following:
- Boot time of the raspberry pi 4 is actually longer with SSD w.r.t. SD card (at least it was for me)
- Load time of SF2 sound fonts was quicker, but not my much
Do share your experiences though, it’d be nice if the RaspPi learns to boot from SSD a bit quicker 