qcrisp
Grimscribe
I think the -ess suffix for femininity is a pretty one that would be a shame to avoid. Although there are exceptions, like 'manageress', as you say, that is a bit of a mouthful. (I think the "-trix" suffix ugly by comparison).
Perhaps because of its bizarre angularity, I rather like the '-trix' suffix. Possibly another attraction, for me, is the irregular plural '-trices'. If it were generally applicable to words ending in 'x', we would be putting things in bocices, perhaps, which might be pronounced like 'boxes', but has quite a different look. Also, I rather like the letter 'x', which is more of an ornamental embellishment to the alphabet that a utilitarian necessity. A box could perfectly well be a bocks, but it would be sad day that the substitution was made. People (not just me) seem to like the 'x' so well that it is assigned the additional duties of signifying a kiss, marking the spot on pirate treasure maps, and so on.
All of that said, I use (I'm sure) the suffix '-trix' very little in The Warriors of Love. In fact, there is (I believe) only one pre-existing '-trix' word to be found in the entire series. That is 'genetrix'. It means 'female parent' (the male form is 'genitor'). The word isn't my invention, but seems handy for a female parent who is not the mother. I think though, that the word is only written in full as 'genetrix' a time or two in the entire series, and not at all until Volume 11. Contracted as 'genny' the word is important and used frequently from Volume 3 Daisy onwards. Daisy's genny is Modesty Clay.
There is also a '-trix' neologism which first appears in Volume 2 Margaret, but possibly doesn't figure in any of the later volumes. That is 'Surretrix' which means the same as our 'lesbian'. An association with Surrey, rather than Lesbos, is appropriate for these books.
Talking of feminine endings, particularly with 'trix', I like 'executrix'. This article lists some others:
http://www.cjr.org/language_corner/womens_suffixes.php
I hadn't heard of 'aviatrix' before.
Another word of feminine connotation and specialist usage that I quite like is 'matriculate', which apparently has roots in 'matrix'.