So I dig into this and 2 things were determined.
1) My AV receiver was not doing a true pass through even when all scaling/conversion options were disabled
2) When connected directly to the TV, 3x and 4x work but the resulting image is fairly dark regardless of the connected system. 5x doesn’t work, but I think the TV itself is the cause of that, as it doesn’t seem to support the needed input to handle what the OSSC is sending.
For #2, running it through the HDMI input on the old generic $20 SCART to HDMI scaler that the OSSC was intended to replace seems to have resolved the brightness issue with 3x and 4x and brings no noticeable lag.
Case closed: AV receiver and TV suck 🙂