I know the general rule is take off 1/3 - 1/2 of flowered shoots but I'm afraid if I do that it will be rather bare as there were so many flowers on it. Is it necessary to prune more/deadhead all the flowers or ok just to keep it tidy?
Never heard of that general rule and only cut off the odd branch on my own ceanothus when it is growing into something I want to encourage or save from being overgrown. They have got bigger over the last few years but not regularly pruning or deadheading seems to have done them no harm in my own garden. I think I read somewhere that they don't like being pruned.
I think it depends whether their evergreen or not.