Timing to Prune Euonymus Red Cascade

Wondering if anybody knows the best time to prune the above.
I have grown this large shrub/ small tree now! for over 25 years and never pruned it. Every year I think I must and then promptly forget about it.
It is now far too tall where it is and needs a good 5 ft taken of it which is fairly dire.
I am presuming like most deciduous shrubs it is best to prune in the dormant period? Living in a pretty cold part of the UK I tend to prune quite last so as not to encourage new growth too early. It obviously flowers in spring so if I prune now I will loose the pretty insignificant flower but not fussed about that. All I do not want to do is kill the shrub as it puts in a wonderful display of autumn colour, the reason I grow it. Bud break will be happening in about 10 days so thought maybe now would be good? I could maybe delay prune, doing half this year and half next?

 

Many thanks @shiney. Mine is not that bushy as never been pruned so no fear of upsetting the birds. Oh I was so upset a few years ago as was hacking back a huge Holly only to find a Robin’s next which had eggs in it. I immediately stopped and stuck back the cut branches to hide the nest again. I sat for ages a distance away from the nest to see if the Mum came back and she did so all was well. 5 babies were raised. It was early in the season and I did not think Robins would have started breeding but they must start early!