The corpus cavernosum urethrae, better known as the corpus spongiosum, has different characteristics that will make it difficult to get larger or harder.
First, from a physiological and reproductive standpoint, it would be counter productive to perpetuation of our species for the CS to be a high pressure erectile chamber like the corpus cavernosa. There would be compression of the penile urethra and cause retrograde ejaculation into the bladder (just like for many of us guys who take drugs like Flomax for prostate enlargement, but causes retrograde ejaculations for different reasons). The net results is little to no semen. You don’t increase the population when this happens.
Second, as your research probably showed you, without the rigid tunica investing it, the CS acts differently than the CC. Once the CC’s become filled at erection, the rigid “box” which the tunica forms, compresses the veins that drain it causing the pressure in the CC to rise to several hundred millimeters of mercury (mmHg). Normal blood pressure is an average 120 mmHg. This makes the CC very hard. The CS has continued blood draining through open veins during erection, keeping it spongy (I think that’s why it’s called the corpus spongiosum). By the way this is not the venous leak that you probably have heard about. This is normal physiology. The distal (or far) end of the CS is the glans. As erection builds, the blood flow increases as well, but until complete erection, a lot of blood shunts from arteries to veins (to be technical it acts like a ateriovenous fistula) which then drains from the penis. This keeps the glans less than engorged and not that hard. At near complete erection, the increased blood engorgement occurs, there is blood congestion so that blood drainage decreases and the head reaches its erect size and hardness.
So what can you do? You already use a ring. This overcomes the venous drainage and allows your CS to engorge completely. You may find that you don’t ejaculate well, because of urethral compression (ring vs. spongiosium hardness?). This as you state has no permanent effect. Jelqing may increase CS size, but not to the same degree as CC because of the absence of tunica. You will not be able to generate the pressure in the CS venous sinusoids to cause as much microinjury in the trabeculae (the walls surrounding the blood in the erectile chambers). This may increase its size at full erection, but as you said you observe, your CC will become very hard before your CS will become fully engorged. Pumping , squeezes, slinkies, etc. will probably do no better.
My suggestion: Continue girth type PE and wear the ring. At least with the ring you will better see the extent of your PE success on your CS.
Good luck.