Thinning this plum tree

[Monvelo]

This is a Mirabelle gauge tree I think. If I take off all the small twiggy branches myself, would that be a bad idea? Assume it'll go nuts and I'll have to do the same every year. I would like to let some sun through as the garden is 100% shade. But I don't want to keep up the brutalist pollarding that was done before.

Sorry I can’t help I wish I could, watching with interest as I have a huge st julien in my smallish back garden and I’m clueless.

You need to be really careful of you prune now as it's bird nesting season so you need to check you aren't disturbing any nests or you can be prosecuted. After August is ok.
If you prune in growing season you will lose all your fruit and it will encourage it to grow more. If you want to prune, I would wait till winter.
What are you wanting to achieve because it will always grow back to that sort of canopy coverage as that is just how that sort of tree grows.
Would reducing the overall height help? So it cast a smaller shadow?

Yeah I know about bird nesting re wildlife and countryside act, I've checked and it's clear. It's better for the tree to prune it in summer., I believe that's the thing with plum trees. It never fruits well, as in a handful only. Lovely blossom in the spring though.

I'd like to achieve it just being a bit thinned out, so the shade is more dappled. Height doesn't bother me.

To thin out your tree you should try to remove no more than a third of branches, if not fewer. Remove the most vertical shoots or those that are double/ triple or crossing / rubbing others. Go slowly and monitor your work by checking it a distance. There is no hurry and you can always remove more branches later.

I thought fruit trees need pruning outside the growing season?

Stone fruits need pruning whilst in active growth, so now is ideal. It’s to avoid various diseases including silver leaf.