I have never tried the suppository route before, but have given myself something very similar to a glycerin "suppository" followed by a soapy enema by creating a layered enema in an enema bag.
Although glycerin is soluble in water (or a soapy water enema) you have to take a little effort to mix them together. This type of enema is taken by not premixing them in the bag (or bucket). My typical volume for this is 2.5 liter. I usually limit this enema to a 3-liter maximum volume with an "average" of 10% strength by using my lower colon as the mixing chamber.
First, I add 250 ml of 100% glycerin to my bag and allow it to fill the volume of the hose. The glycerin is relatively thick and takes a while to drain out of the measuring cup (I buy glycerin by the gallon) and down the length of the hose. Once the glycerin reaches the end of the hose and nozzle I close the clamp.
Then using the measuring cup that still has a residual glycerin, I make up my soapy enema "top layer." A total of 2.25 liters of warm water with 30 grams of shaved Ivory soap dissolved in the water is carefully poured on top of the glycerin layer. There will be some mixing at the top of the glycerin layer. But if you pour it in slowly the mixing is minimal.
I use a balloon retention nozzle (and sometimes an inline Higginson pump) to take this enema. Once the nozzle is inflated and secured, with the bag suspended high, I open the clamp to the fully open position. I depend upon the mass of the water on top of the glycerin layer to provide sufficient pressure to have the glycerin flow out the bottom of the bag and flow through the hose. Although some of the glycerin clings to the walls of the bag (and begins dissolving into the warm soapy mixture), most of the glycerin is the first down the hose and into the rectum. In my case, the glycerin doesn't burn even though it does cause my rectum and lower Sigmoid to tingle as it coats the walls and slowly flows up my rectum, Sigmoid, and into the descending colon. The glycerin is viscous enough to take 10-15 minutes to flow out of the bag and hose before the soapy enema solution (with the residual glycerin now mixing in with the enema solution) reaches the hose and then begins to flow into my colon.
Things speed up considerably once the thinner soapy enema flows into the hose and begins to sweep out the glycerin remaining in the hose. Once the soapy solution reaches the hose and the nozzle, the rectum and Sigmoid become the mixing chamber and then the resulting enema ends up being a variable glycerin strength soapy enema that is of a sufficient volume to reach well up into the transverse colon. The glycerin has one effect and the soapy enema glycerin mixture has a power effect further up in the colon.
It takes the balloon catheter to hold it all in initially but after holding the solution for a short period of time, if I remove the balloon nozzle, my rectum closes down to such an extent that the enema effectively becomes a self-retaining enema. The enema will gradually be forced out with intense muscle contractions and all the actions associated with the dissolving and churning of the enema, but it isn't such an easy flow out. Moreover, inserting a simple small nozzle to begin the rinse process is a challenge because the rectum is so sensitized to physical contact and so closed down that getting the nozzle in is a challenge.
Eventually, the rinses will begin to flow and allow the original enema to flow out and begin to mitigate the effects of the soap and glycerin. This doesn't happen over an hour. It takes several hours (like 6-8 hours) to complete the enema process and end up well cleaned out.
This enema has the advantage of being all liquid and so the glycerin doesn't have to melt the way suppositories do.