I have confirmed that the ER-X can indeed do up to ~250 Mbps with SQM turned on!
I also revised the TM-AC1900 Method page and added instructions on how to turn on HW Acceleration. The TM-AC1900 is unique on the Merlin firmware because it can still use fq_codel with HW Acceleration on!
For future plans I may add some screenshots and do a write up for the ER-X setup. I still feel like the YouTube video by Battle(non)sense is solid because I was able to follow it and turn on SQM myself.