I can’t tell if the ping increase is due to your wifi or internet connection being choked. But usually when it is your internet connection, the ping increase is a lot higher than that. Have you considered running a wired Ethernet connection but using wall hooks/strips to tidy it up and keep them away from people/animals? Do you have coaxial cable in the walls already (for TV)? If so, MOCA is another good option. Powerline networking is possible, but it is an absolute last resort as it has drawbacks.

They can just keep changing their minds about what they put inside as long as it meets the level of performance advertised.

I would just make sure that I don’t store anything important on it. But since it is so big, it can even double as a backup drive for the main system.

QoS and traffic shaping is the best way to manage this kind of situation.

Make sure that real-time (latency sensitive) traffic has priority, or even just do something as basic as throttling certain ports/devices on the network.

Just keep in mind that you’ve mentioned the download speeds, but upload is something to consider as well.

Worst case scenario, keep mobile data on your smartphone as a backup.