Don't take one person's opinion too much to heart. Nobody can offer perfect solutions for everybody, so naturally some people will always find something to criticise. You have to live with it but they have, too. If they can't, it's their own fault.
And as long as they don't make good suggestions as to what you could improove, they have no right to criticise you anyway.
Excellent idea! I should have a trash day, too. So many things accumulate and it's a relief to get rid of them.
The same appears to go for problems, particularly the accumulation thing.
I agree with your union. Wait till the interview with HR and don't mention your headspace problems - they are irrelevant when it comes to your work. Give HR the facts that you gave us a week ago: that you performed well, and never received any thanks but no negative critics either so that one must assume he was fully content with your work. Tell them that your boss, in spite of having the office next door, didn't contact you before he accused you with HR.
The main strategy is to show that your boss never showed any miscontent with your work and that his late attack came out of the blue. It might be good to hint that his unexpected attack might possibly be connected with him leaving in April (the stress of moving, finding a new job, having big changes in his life etc).
I'd recommend that you discuss the strategy in detail with your union so that both you and the union representative will say the same to your HR. A point usually gets far more weight with the listener if 2 people agree in every detail.