If you have never done it before then I really wouldn't have the first time you do be with a client.
I absolutely agree. Or with a caveat that I would do it with a client who I had been seeing for years and trusted to be sensible (and who had a small-to-average dick)
Yes an anus is less flexible than a vagina. It doesn't produce lubrifaction like a vagina does, so you need to use a LOT of lube, and preferably lube which is specifically for anal. It's a lot more common to get an anal injury than vaginal (through sex).
However it's a total homophobic myth that anal sex causes sphincter damage if you have a sensible partner (or are one.)
What you have to be careful of is porn hounds who think they want to "destroy your ass" (why?) who have no idea how to "warm up" and just want to stick it in and fuck like a pneumatic drill. uggggghhhhhhhh.
I get a lot more enjoyment fucking guys (carefully!) up the arse with my strap on 😜