Whether you expel in the shower is a personal decision. Is it messy, of course! But it all goes down the same drain. I use the shower hose for cleaning my rectum before anal play, but I don't allow the water to go past the sphincter and into my bowels. Any further or deeper cleaning almost always results in unexpelled water leaking out at very inopportune times.
If we're doing enema play, then I clean my rectum first, like above. Diana will finger my anus and massage me to relax and open me - and sometimes insert a toy before finally administering the enema and stroking my cock while the water flows until I orgasm. Then I savor the peace and relaxation of the release on the toilet before returning to bed to cuddle and pleasure her, but after the enema release - that's the end of our anal play.
There's no need to do a full enema before anal play, unless the anal play IS the enema.
Here's a couple of fittings that we've attached to our shower. The first is a 3-way lever which directs the water flow to the shower head, to the shower head AND the attached hose (this is the setting I use while cleaning my rectum as the water continues to flow out of the shower head keeping me warm), or to the hose only (that setting is usually used to wash anything unwanted quickly down the shower drain).
https://www.homedepot.com/p/Delta-3-Way-Shower-Arm-Diverter-with-Hand-Shower-in-Chrome-U4929-PK/203155698?cm_mmc=Shopping%7CB%7CBase%7CD29B%7C26-3_FAUCETS%7CNA%7CPLA%7c71700000033603010%7c58700003907844571%7c92700031447644991&msclkid=f7f1916fd50a1f6b6a0fcc0d30c2fc50&gclid=CPKQh_vknuQCFUFMgQoduo0KUg&gclsrc=ds
The second attachment goes between the shower hose and the anal nozzle. By moving the lever back and forth you can control the strength of the water flow from really hard to a slow trickle. It works great!
https://www.amazon.com/Control-Pressure-Regulator-Restrictor-Sprayer%EF%BC%8CUMIRIO/dp/B07LCR71XV/ref=sr_1_11?hvadid=78271538109339&hvbmt=be&hvdev=c&hvqmt=e&keywords=shower+flow+regulator&qid=1566761329&s=gateway&sr=8-11