Iron I think is usually applied in areas with alkaline soils, such conditions are not likely to be a problem if you are here in the PNW.

Spotting could be foliage mildew, this became common in my area some years back and causes leaves to drop off prematurely, resulting in gauntness - even death with most susceptible kinds of rhododendrons, such as 'Virginia Richards' and 'Unique'.

I'd like to see a picture of the root area and trunk.
I think that's where the problem might lie.
How much sun does it get?
Mike

Yes it gets a lot of sun at the root area. Others have suggested thick mulch to protect.

Since it's lived long enough to get that big it must be in a halfway decent spot.

Rhodies like a lot of organic matter in the soil, and water in summer as you've tried already. The leaves do look like some kind of mineral deficiency but you've already added iron and magnesium. I'd give it a nice mulch of compost. Not right up against the trunk, that would rot the trunk, but a few inches out from the trunk, and then out at least as far as the branches extend. It doesn't need to be more than a couple inches thick so long as you add more periodically. Better to add more and keep it thin, than to bury the poor thing in several inches all at once.

Rhodies have very shallow roots so they are very sensitive to changes at the soil surface. I've found that I just cannot cultivate under rhodies at all - not even delicate little scratching to cultivate in the fertilizer. Leave the fertilizer laying on the soil surface and mulch over it. Do not use any herbicides under a rhodie. Water the top 6-12" of soil but don't worry about soaking it any deeper than that. Make sure it has good drainage - it wouldn't have gotten this big with poor drainage, but the drainage could have changed over time.

They don't need much fertilizer usually so don't keep feeding it over and over. Once or twice a year, in spring, with a mulch of compost, should be sufficient, that's all mine get. If there's root rot or root weevil damage it won't be able to pick up the fertilizer no matter how much you add.

We bought a property with about 35 rhodies that were at least 20 years old and as far as my experience (over the last 8 years) they aren't that picky. In fact, the ones that were the oldest and straggliest (sp?) we simply cut all the way back and, although it took about three years, they came back better than ever.