I noticed you asked this question a while ago and I wanted to confirm your question with some answer specific to what you asked.
Running SampleTank 4 Max on Windows 11 in REAPER 7. Been using REAPER since Version 5.
i5-9400F - 16GB RAM - 1TB SSD (for running programs) - 2TB for storing sound banks and samples. (Time to buy a newer system after the holidays).
I have not experienced any issues running any IKM programs in REAPER. I have only experienced crashes when I get impatient loading samples and start trying to do other stuff before letting everything load. That causes problems with anything though. I also use NI, UVI and Arturia plugins and the same thing happens, so it's not an IK problem.
I don't know what the benefits of using VST2 versus VST3 is. I have begun using VST3 for everything because I don't know when Steinberg/Yamaha will pull the belt tighter on VSTs and we find ourselves struggling to work with the old format. So I use VST3 whenever it's available.
I've had no problems using the two together, as I do use some Open Source stuff that only comes in VST2. I have not experienced the same issues that some people have reported with VST3s.
Regarding track work: I typically run ST4 as a single module and route tracks to it as needed. I would rather run it on each track separately, but I try to be nice to my CPU because I often have other things running on other tracks. MixBox, TRackS5, Amplitube, _, Toontrack, Hammond B-3X, etc. LOL!
So... to try and make sure that I don't overload the CPU\GPU, I keep an eye on that meter. But that is me and my compositions can become rather track intensive. It gets crazy in here. However, for some of my smaller stuff; I have run a couple of tracks with separate ST4 instances. Truth be told... on two of those, I did end up consolidating them down to one ST4 and using individual outputs in REAPER to process them separately.
My advice on that would be "You do you." Whatever works to blossom your creativity is what you probably should do. My ideas come and go very quickly, so I often will use individual instruments to tracks just so I can get my idea down quickly; and then come back later and sort it all out.
Well, I hope better late than never and helpful. I have found that going digital has really stressed my workflow. There are so many options and choices that I have lost a few ideas to just getting going. Not like the old days when my biggest interruption was striping my sequencer to a tape track before getting busy.
Good luck and happy creating!