I do think a measurement mic and REW is the best way to go to get improved sound. Actually, consider that if you post an REW measurement file here, people will do the work for you. And likely better than many pros out there (due to instant peer review of suggestions).
That said, I will give you my take on gear, since that's what you asked for. Not that I am an expert on all things gear.
Short version: you have very good gear, and any new product given your budget will give you improvements you might hear, or might not. Even with FAR more money improvements are going to be VERY slight from this point on.
Having used an E30 ii for a while, I recently got an SMSL do100, since I wanted balanced connections for the amp I just ordered. Comparing the Topping and the SMSL using single ended connections, the difference is pretty small. As in, I need to A/B to really hear that slight difference for sure, but just listening on first install.... maybe better, i think better, yup better a tiny bit, maybe?
I do think a streamer is a good upgrade. Wiim is good enough. Depending on your PC, since those can introduce noise, it might clean up the sound. But mostly it is a convenience thing for me to be able to switch things from my seat. Though the Wiim eq/peq options are nice to have, and fun to play with, even without measurements.
Speakers? I am sure if you spent a LOT more money you could get small improvements in measured performance (hearing that is another matter). But for me, I like a bit of variety. So maybe another pair of speakers to swap out to change things up? Something with horns, or an AMT or ribbon tweeter? I did recently look for sub $2000 USD speakers, and there are a lot of them that are very, very good. But different.
I don't think any change to the amp or preamp will give you any difference you can hear, unless it is negative. But, maybe a second amp, something with tubes (if you like that) or something with a "warmer" but no tubes like a Cambridge or something along those lines. Measurement downgrade, but maybe a good change of pace once in a while. There are a lot of used amps out there as well. .
If you stop thinking "better" and start thinking "different" then you will have a lot of options to change things. That might give you more enjoyment. Though I say that as someone who has 2 amps and 2 pairs of speakers on switches, so of course I think this way. Mostly I play one system, but at least once a week I switch things up. That helps me to enjoy my accurate system more, really. My other system is not accurate at all, but it is fun.
Enjoy!