….put a couple of red bricks into the tank to take up for less water ….
Will that work? Ish. The problem is, the size of the flush is directly relative to the size of the water in the bowl. So - if you have a huge bowl with a lot of water in it, doing this might mean it … doesn't flush quite fully, and you end up with dirty/smelly water left over. There exist kits designed to lower the float and use (slightly) less water - they will have less of an impact, but even then, the correct solution is really a new toilet.
To the original question though:
how do those of you that have them deal with the issue of large volume enemas?
Just like anyone else, and the reason why has to do with how toilets work. A toilet is nothing but a water bowl connected to a vertical, inverted U-curve bend, with the top of the u-curve set at just above the desired water height in the bowl. The u curve, once the water reaches it's bend, doesn't allow the water in the bowl to flow into the sewer, while similarly forming a water (and thus air) tight seal to prevent the smell from an open sewer pipe to escape. Genius, as it were, and ZERO mechanical parts.
Placed above all of this is a water reservoir with a valve. When you open the valve (which is what the flush handle does), all you do is dump the entire contents of that reservoir into the bowl (that's where you see water coming down the sides of the bowl). That causes the water level to rise above the U curve in the bottom, which then starts pushing water through that U curve, since the water level rises above that bend. That action creates lower pressure in the bottom as well, which helps to pull water that was in the bowl (which is now, largely, lower than the water dumped in from the top), down the bottom of the bowl and through the U curve. The water dumped into the bowl continues until the reservoir empties, at which point the valve closes, and the bowl stabilizes at the level of the U-curve again. The reservoir then fills, which is why your toilet still makes noise for some time after the water in the bowl seems to be settled, and you can't fully flush again till that stops.
Interestingly, if you dump a large amount of water into the toilet, from any source (say….an enema), this process happens similarly. If you were to say, dump a 5qt enema into a 5 qt toilet, as you expel, you will sortof … flush the toilet while you do. The water from the enema will raise the toilet bowl above the U curve, and water will exit the back end (and then down, into the sewer). Since the water you're putting in is (probably) dirty, the clean water at the bottom is flushed first, leading to more dirty water in the bowl than clean. Since you will (probably) not expel water as quickly as the flush mechanism, you won't get as much of (if realistically any) pressure effect from rapidly draining water, but it will still drain.
If you take another enema, and expel it without flushing, you'll find that the water likely gets dirtier (increased dirty/clean water ratio), but actually… doesn't rise. In fact, if you do this ten times, you'll find that you can't overflow the toilet. If you want to further confirm this, get a garden hose, put the end in your bowl, and turn it on. The toilet bowl will never really fill, and definitely not overflow. Toilet overflows are not caused by normal operation, but by blockage of the drain line.
No matter how many times you do this, if you then flush the toilet, the same action as normal occurs. Even low flow toilets are designed to replace the entirety of the water in the bowl with each flush (for sanitary and scent reasons), and so the remaining enema water is then flushed.
Lower flow toilets tend to have smaller diameter u-curves and/or lower pressures in those curves, so they sometimes have more difficulty clearingh solid or simi-solid material (large stool, etc), and thus, more frequently require assistance (plunger - that's all they do - manually increase pressure behind the blockage). They also cycle less water through the bowl, which sometimes results in increased cleaning needs. Enemas, however, are largely liquid, and pose no difficulty for a low-flow toilet.
Keep enjoying your enemas in full confidence that your toilet can totally handle it, while also knowing that you are, in fact, saving water (if nothing else, saves you cash each month!).