You can get a ounce per month but you should be able to get more then a oz per month of grow time.
Had a similar size room long ago same wattage bulb also.
But I had no additional veg area and I pulled down 9oz+ dry weight every 4 months (1 month veg 3ishmonths flower).
9oz Was my lowest yield the largest was 1lb+ but the quality was lower. One reason for the lower quality was genetic (the seeds used..the strain) The other was that the 400w hps didnt have the light penetration to get to the lower buds.
With no rotation of plants (all out at once)
Looks like you need to change method a bit (more plant training).
I agressively topped my plants. Look into things like......
Main-lining
https://www.rollitup.org/subcools-old-school-organics/542308-main-lining-thread.html Amazing but takes a bit of time to do.
SCROG
SOG
super cropping
lollipoping
FIM ing.........
With a small room and a low wattage HPS its all about canape management. You want to keep your plants short, wide and as even in height as you can.
That way you can keep the light as close to the plant as you can not just the tip tops of it.