Don't over analyze it. It's a shrub. You can prune it. Flower buds are formed at the end of the growing season so now, right after rhododendron finish blooming, is the time to prune them.

The question becomes and a picture would help, what does quite leggy look like and actually mean? Depending, they may need more actual pruning for branching lower down than pinching of terminal buds will accomplish.

Thank you! Maybe leggy isn’t the right term, but they aren’t full. Here are some
Pictures….

i think you are considering the wrong type of pruning ....

it looks to me ... they need some renovation to create a whole new plant lower down.. rather then pinching.. which will make a bigger shrub higher up ...

it is easy to prune.. when you can put into words.. what do you see the plant being in 5 years ..

e.g. that one plant in front of the window ... do you want a thick shrub covering the whole window .. meaning higher up .. or would you prefer a shortened but fuller shrub more in line with the window height ...

watch a few of the vids below .. and see if you can come up with a plan of what you actually want to accomplish ... and then we can offer guidance on how to get there ...

and understand.. the way these things grow.. a full and proper renovation could take a few years to accomplish ...

and to be sure.. if you just want to give it a haircut and bush out higher.. all the power to you.. they are your plants.. around your castle ... and who the heck cares what i think ...

ken

https://duckduckgo.com/?q=rhododendron+rejuvenation+pruning&t=ffcm&iax=videos&ia=videos

two more thoughts ...

first ... Rhodies and azalea are the same rules .... so you can watch those vids also .. the more you watch.. the less stress you should have deciding whatever you end up doing ....

second ... if you deadhead just the flower.. often.. if not always.. that is where the new branches branch buds will appear ... i hope you arent cutting them back too far when removing the flowers ...

ken