Upgrade from AC3000 to AX4200 or AX6000?

Will you see real world improvements? Potentially. The AX systems are much more capable when you're getting up to the 1 gig speeds. 

But is that going to make a difference? Its tough to say. video calls don't take a ton of bandwidth. Its just that its a live stream as well as you need to upload. Most of the cable systems advertise 1gig speeds but those are download speeds. Their upload tends to be a fraction (usually 30-40mbps) of that. So the issue might not be the router setup. 

What I'd try doing? Buy from business in your local area that has a decent return system. I know best buy/costco have pretty good return policies. Try it out. See how well it works. Its a heck of an investment if it doesn't provide any actual benefits. If it doesn't help, return it and add another satellite to your current setup. 

Any chance you MIL suite is hardwired in? If so, you could always use the new system in your home and the older orbi setup in the MIL suite (in AP mode)