That’s a tricky downgrade, wouldn’t take 1.2.1 directly, SOS10, I was able to go to 1.1.1 but only briefly before an OTA update kicks in and ends up back with an SOS10 after a multistep update process. Safe mode is either broken or what’s doing the SOS. 1.3.1 was the oldest I was able to go w/o SOS and it had the same SGP issue as the newer firmware versions.
The issues I encountered and steps I took can be seen here: How to downgrade firmware