Kiki, a perfect tryst.
Mistress has BDSM connotations that keep it out of my usual verbiage. Tryst, lover, girlfriend, fuck buddy, friend with benefits, side girl, side action, paramour, concubine, inamorata... those are all fair game but if she breaks out a whip or tries to cuff me... we are done. Careful using the term mistress. It can mean a lot more than just my fuck buddy. Although not specifically related to sex in the definition, the BDSM variation of mistress is actually the first thing listed for the word in the Oxford English dictionary. The second listed definition refers specifically to an extra marital affair. You may wish to rebrand that term to Lover or Friend with Benefits because most folks here would struggle with paramour and inamorata. Not digging the term mistress, my friend. Both standard definitions of the word do not apply here because the ladies are all aware of the others, there is no marriage and many of the girls actually know each other and are friends.
Quote below courtesy of Oxford Languages...
mis·tress
/ˈmistrəs/
noun
1. a woman in a position of authority or control.
"she is always mistress of the situation, coolly self-possessed"
2. a woman having an extramarital sexual relationship, especially with a married man.
"Elsie knew her husband had a mistress tucked away somewhere"