I'd want to confirm that there is something being maxed out on the router (CPU, throughput, etc) before spending money on a replacement. TP Link makes decent routers but in my opinion not as good as the Asus and it could end up being a downgrade. The router could be dying but you'd probably be seeing more symptoms than just one person having issues.
I'd say to start, upgrade the AC3200 to the latest firmware (if not already) and do a factory reset and reconfigure from scratch (not a backup). That can solve some "unexplained" issues and is a good idea to do from time to time. If that doesn't help, it is more likely an issue with your ISP, her PC, or wireless signal quality, as that is a plenty powerful enough router (unless you're also doing lots of other stuff at the same time, in which case you should look into using QOS on the router).
Daisy chaining her off two routers if anything would cause more issues not less in most cases. The exception is if you have a lot of IOT or other devices that are very "chatty" between themselves, putting THEM on an isolated network could help, but I'd leave your main stuff on the main router.
Using Guest Wireless 1 on the latest firmware is a good way to put your IOT devices or stuff you want isolated onto its own segment and reduce its impact on the main network. GW1 gets its own subnet and can either be partially or totally (access intranet disabled) isolated that way.
Since you already have two routers you could try daisy chaining them and using one for the isolated IOT network but don't think that would buy you much over using one router as your main and one as just an AP to extend your coverage. I'd do some research into which is the more powerful/reliable router (not sure off the top of my head) and just use that as your router, with the other as an AP if you need extra coverage, or as a secondary router for your IOT/lower priority stuff if you decide you want to try that. If you use the 85 as your main router I would also upgrade that to the latest firmware, factory reset, and reconfigure it from scratch.
You could actually use the 85 as a test to see if it helps her issue, get it all reset and reconfigured, then swap it in place of the 3200, see if anything changes. If she still has problems then you need to start narrowing it down, maybe unplug any smart home stuff you have, watch the bandwidth usage while she's on a call, etc. You can try hardwiring her to the router to see if that helps, if so you know there is a wireless signal issue. It could be as simple as some process running on her PC that is interfering, or a wireless driver was just auto-updated and is causing problems, etc.
There are test sites where you can look at the quality of your internet connection for latency, jitter, packet loss, etc all of which affect voice/video quality.
Heck, assuming she has a decent firewall on her PC (windows firewall is fine as long as it hasn't been messed with) you could plug her directly into your ISP device (modem or whatever), reboot it, and see if her problem is still there. If it is, then you've just narrowed it down to either her PC or your internet connection.
Don't see anything that having a 3rd would get you, unless you need more coverage. The "main" router in that setup would be severely underused and a waste of money.
For reference, I have an RT-AC1900 (similar to the RT-AC68U), quite old, and my 350/350 internet connection can be maxed out no problem and I do Zoom and Teams calls all day (over wireless). I don't have a ton of devices on my network though, and nothing IOT other than a Fire TV stick.
Long story short, in networking, KISS principle usually is correct, adding more complexity probably isn't going to be the solution.