The best antenna right now to use is the Peplink Maritime 20G antenna with these RP-SMA Male converters. You could also use the Peplink Mobility 42G Combo antenna if you wanted both WiFi and LTE. The 42G is not as good as the 20G, and it is smaller, but it depends on your space and desires.
Probably the best way to deal with this is to leverage things from my Managing internet usage on your boat article and unfortunately some manual intervention. In particular I would look at QoS and bandwidth control. You could set the non critical devices to have hardly any bandwidth percentages, and the important ones with higher percentages. You need to make sure that your internet connections have correct settings for their upload/download numbers so those percentages work, though.
I do something similar when I’m connected to remote WiFi, and for updates, Dropbox/OneDrive, and the like, I use the metered connection settings in Windows 10/11 to manage it. I also use Low Data Mode in iOS and turn it on and off depending on what I’m connected to.
There are more automated ways of potentially handling this, but it requires some more complex firewall rules along with priorities and such to try to catch the “bad” traffic when using the slower connection. The challenge is that systems like OneDrive and the like can change how their traffic looks as they release new features, and figuring out how to block it or slow it down can change quickly.