If you've got email evidence of all of this, I'd raise a sex discrimination case with HR. Advising it's not a grievance yet, but you want this sorting.
I would factually lay out the timeline for it.
eg returned to work on x
asked for work y
state prior to maternity leave you were covering projects of x level, now being ignored on being given projects and asked to do z tasks.
Attach the emails in order.
Give evidence you have asked for feedback on each task you've been given and completed. Attach evidence from others where they have given positive feedback.
At this point you've got not a lot to lose, so I'd fight fire with fire (so long as I had evidence).
I've been at one company for over 25 years. Had a pants manager, we are supposed to have clear objectives set at specific points in the year, he was useless and didn't, nor did he do regular reviews and document them. I asked several times on email, but to be fair didn't really push. but did (luckily) have an email trail that showed I'd tried. That Nov redundancies were announced and if it goes to formal, they review your performance based on objectives. He sat me down in Dec and told me I'd failed my objectives that year, and I was being marked down (his mates though all got marked up!). I emailed HR and him, stating it wasn't a grievance as yet, but I'd had no formal objectives throughout the year despite me asking, nor formal reviews and I had evidence that I had tried at least half a dozen times, as such I was not accepting the lower grade at year end, as I'd not been told once there was an issue. HR agreed with me and I received a "good" PDR and rating in line with what I expected that year.
Sometimes managers are just crap and if you have evidence to support your cause I would raise it.