I haven't kept up with the the latest plugins. Do any of the latest API plugins have the weight of the HW? (Or for plugins in general do you reach for separate plugins for saturation/harmonics?)

I recently picked up a Bettermaker v1 limiter and I'm comparing to software limiters. I'm leaning toward keeping the Bettermaker, because it seems to allow me to push it further than plugins and it does something I'd like to keep. (I'm not sure if that's $1500 worth of something. So, I'll be testing more.)

My next addition would have been channel EQ's for tracking on the "money" channels, but I think I'm getting by ok without them. So, I'm wondering if there are any plugin developments I might have missed that are must haves.

Yes. A pair of those 550b modules.
Hmmn...

The Pulsar Audio MDN api plugin was it and a bit awhile back.
I haven't tried it yet myself, but there's plenty of good feedback on it.
Might be worth a try
https://www.pulsarmodular.com/product/p455-mdn-sidecar/

My 13” MacBook Pro with the api plugins on it weighs about 4 pounds, so probably not the same weight. I do like the sound though when audio runs through the plugins.

I have the Waves and UA offerings and to my ears they don’t have the same gravely sizzle. I haven’t used the unison, but there is something to performing into nice circuits. If you’re monitoring the sound as a performer, you will respond to what you are hearing and perform differently. If a preamp breaks up in a good way you will sing differently than if it breaks up in a nasty way. Hardware has its advantages.

I have the MDN plugins. They are probably my favorite API flavor of the moment, but I haven't checked any others outside of Waves/UAD.

I think I'll eventually pick up a pair of 550a's and some 560's. (I like 550b's as well, but I'd start with the others.).

I may end up getting a pair of general use channels like SSL's and a Pultec as well.

There's a few hardware pieces I still want, but my list is way less than it used to be. I'm loving what working hybrid brings to the table. But, I'm not desiring HW the way I used to. I'm probably 85-90% ITB/10-15%HW

Best API plugin I have? Slate Digital's.

https://slatedigital.com/fg-a/

That's the only one I haven't tried yet. I'll have to give it a go.