I don't know the whole story mate, obviously. But I think jumping ship now would be premature, professionally and financially. Take as long as possible with a phased return, you dictate the timescale. Obviously if the mental health issue gets too much, go with your gut.
Take your time with the grievances, make sure you log every single event, lay it out precisely and professionally, mention witnesses etc. Timing is everything. Once done, then you can let the grievances run their course (making further grievances if anything new comes to light, especially if they don't follow policy*).
IMO the financial value is in the sick leave. I respectfully disagree with the union saying wtte
"as I've been paid during my suspension it's unlikely I'll get much of a payout" I was told the exact opposite, though in all fairness it was a long time ago. When they gets a series of grievances in their inbox, then you get signed off, I will be very surprised if they don't offer you at least 3 months for everything to go away.
WRT your union rep, get a second opinion. One union rep who I used, high flyer for NASUWT, later became CEO or thereabouts at a local borough council. Some of them know which side their bread is buttered.
* make sure you familiarise yourself with every single policy and be a total

end. If they stray from any policy, even slightly, file another grievance.