The workaround would be to try the ROM without downgrading to PG1 stock tar.
Your bootloader upgraded; likely for security reasons, downgrade is not advised. I wouldn't attempt that which would mean no full tars or bootloaders below QI5 would flash in Odin. Attempting may activate factory reset protection.
There might still be compatibility with PG1; you can report back any issues but they wouldn't be supported.
FWIW, I'm running QI5 rooted with Beastmode r60 kernel and Xposed. Haven't confirmed any incompatibility but I did have a problem hearing a HD Voice conversation. At that time I was still setting up ROM after just flashing a sound mod and I've yet to have another rare HD voice call. Maybe there's a chance for compatibility with this ROM.
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samep, thank you for this advice. It worked!