Mellower strings for Strat with SS frets?

I've been using nickel for decades. Love the sound!

Elixir makes a coated string for electrics but, from my experience, they leave string poop on your pickguard. Maybe they've improved thru the years.

I would go to pure nickel. Note that this only pertains to the wrapped strings. But they are a less-bright tone.

It won’t cost you much to try a set. 😉

If you like your current strings and how they play, leave 'em on there and start using the onboard tone control.

It does exactly what you're asking about.

Yes, AND/OR use a longer cable. 

I'm using standard nickel wound (which are nickel plated steel), not as bright as stainless wrap, brighter than pure nickel wound. With those strings I have not noticed any change in tone with stainless steel frets vs nickel silver frets. How do NYXL compare in brightness to standard strings?

To my ears, SS frets aren't so much brighter as they have a harder attack to them. More snap, less bloom. NYXLs also have lots of snap, so switching away from them probably will help - even just switching to regular slinkies or something might be sufficient for your ears. D'Addario in general sounds slightly more snappy than other brands to me. Maybe try Fender strings? Made by D'Addario, but a different formula? The Fender pure nickel are my personal favorites, among the every-day priced sets. Heck, changing gauges or using different picks could get you somewhere you want to be, too. I really only ever hear the difference in fret materials on Strats & Teles. 9/10 times one of my clients wants SS frets it is in an acoustic because they beat out the cowboy chord positions so quickly. The difference is barely noticeable on those - definitely there, but in a way that I bet most players wouldn't hear it.

If you REALLY wanted to go nuts you could reinvent the instrument around the frets - brass block or something - but that doesn't seem like the ideal solution here.

My only experience with SS frets was with a refret on my #1 Partscaster and I have not detected any change in terms of tone. In fact I've replaced the pots, cap and trem block and to my ears it still sounds like the same guitar. I use Ernie Ball Regular Slinkys and for a time used their Classic Rock & Roll set. I definitely found the latter mellower so aside from tone pot tweaking/amp settings I'd try pure nickel strings.

If trying strings is what you want to do first, try DR Pure Blues Nickel and go a set higher. They compression wind over a hex core, which make them feel slinkier could be bs, but I sense it. I'm usually on D'Addario otherwise.  Correcting...I checked and it is a round core, so I must have mis-remembered lol