Throttle down the resolution of the display?

Maybe look into router-based options for throttling bandwidth to specific internal devices, starting with a throttle of 1Mbps or less to see how that goes.  Many streaming apps, if not all, will automatically downgrade quality to match what they perceive as your available bandwidth.

I think newer verisons of OpenWRT can do this? Plain Linux definitely can.  It'll take some work to configure, though.  Dedicating a separate Wifi SSID for the televisions could help.

Unfortunately, streaming has and will continue to be data intensive and with the push to 4K and higher resolutions will be even more so.  Video in itself is very difficult to compress in general without major resolution loss, especially with today's video formats.  Even with compression, which is already done at the source, you will not see a significant data savings with streaming lower resolutions, unless you go to the point of making it unwatchable.  And if you have 4 TVs that are streaming or streaming capable, it will be even more difficult to control how much data is used.  The simplest solution would be going back to standard television, like antenna broadcasts or cable/satellite type of viewing, which uses no data.  Or a combination of watching antenna broadcasts for the local channels you watch and only stream channels you cannot watch over the air.  Otherwise, see if your router has some type of data control that can help throttle your data usage but even still that will give you limited results.   Or try to find a plan or a service that has more data that is still within your budget.

By the way the old secret menu for setting resolution only worked for some old codecs. As more and more popular channels moved to different codecs, those settings had no effect on them, so Roku eventually dropped the menu to stop people from complaining that they had no effect.

I'd just dump the other TVs before I'd go to 10 days on/20 days off each month!