@SassyWarrior,
I will pass this request onto the team, as we have seen this come up multiple times in the past few weeks. There are new developments coming to HomeShield that may be able to better track, manage, and protect your network; so keep an eye out for any official updates from TP-Link.
I do believe that for Decos, you can configure a bandwidth limit for the guest network; so this may work if you put any devices you want limited on the guest network.
However, I will give a warning on a personal note as these settings stand.
Parental Controls and Per-Device Settings are dependent on the IP and MAC addresses of the devices connected. Unfortunately, there are now a billion and a half ways to get around these restrictions through the use of Private Addressing, VPNs, Proxies, or even in some cases Peer-to-Peer network sharing. As a result, I would recommend configuring the network settings for the device or PC; most devices now have an option for "Metered" Connections, which will optimize the amount of data needed to perform an action. While bandwidths can be limited from some routers, the experience on the device may change as it may not be able to receive enough bandwidth for what it needs.
Either way, the request is 100% getting forwarded to our team for consideration.