Dordes but there’s hardly any pressure release after the first flush.
I’m not sure what this means. Could you place a video of what “hardly any pressure release” is?
With 4 coffees per day, I’d be backflushing the machine every month at least.
The OPV is the least of your worries. The E-61 is all mechanical, so, unless a valve is partially stuck, or sticky, or not fully opening, then there’s no reason why it should not vent properly.
As suggested, the best thing you can do is to give it all a good clean, lubricate it and put it back together - at least the cam and the bottom half of the group.