Well, of the recommendations scattered in the thread so far, I am not sure about some.... And it looks from some of the descriptions and research, I would have to download, install and evaluate a few to see if they had the features requested.... Some don't really say, others say they have side chain HP filters, but don't appear to always support external side-chaining.
Areena is a reverb from what I see....
I have multiple paid options that fit all the criteria. But freeware to recommend, I am not so sure.
I have MCompressor https://www.meldaproduction.com/MFreeFXBundle
and its a winner IMO. I had to go dig it out from under a pile of accumulated rubble since I last tried it back before Mixcraft supported native side-chaining. I had forgotten about it over time.
I also have Cakewalk CA2A, which was offered as a free promo previously and fits the bill for opto style compression. It will sidechain but it takes a little somewhat unintuitive trick to make it work. Be glad to explain it if anyone has it and wishes to figure it out. Its still avail, not free and with the demise of Cakewalk, that ones likely to become obsolete.
All these others.... Not sure.
The Dead Duck compressor (and other tools) looks like it might be worth checking out.
The few of the others mentioned that I have used, are simply because I installed them so I could run other peoples Mixcraft projects. I generally dont use them and have never become overly familiar with most.
5orcery is an interesting twist on multiband, but I am not sure about any side chaining and while it serves a purpose its a bit complex for common compression tasks IMO.
S.LA.X looks like a simple approach. Sidechaining? Dunno.
I have MJUCjr, similarly- no sidechain support, and I believe 32 bit only.
I like much about the TDR Koletnikov, but no sidechaining.
Limiter6, I just cant seem to get it to fly......
Have to look at some of these others as time allows.