I've tried everything, at first I thought it was my wifi modem, cause even with tv off I was getting 1.5 mbs download, got that replaced, and now my tv has turned into a hog, while streaming I get 3.5 to 1.5 or lower off a 25mbs modem. When not streaming I get 20-25 mbs download, It started slowly sucking up the bandwidth and each week it seems to get worse, losing more and more bandwidth, I shouldn't have to choose between streaming shows and playing an online game, this is getting ridiculous. I wish I could change from HD to SD. I tried to switch it to 3.5 mbs from the hidden menu but it keeps changing it back to auto and auto is anywhere from 12 -25 mbs. After a month of this garbage, I almost want to take it back to the store and get a refund.
Some routers have QoS settings which let you throttle how much a connected device can get. But, then you might run into other problems (like slow guide/info if you watch antenna tv). Some streaming apps might detect the slow speed and downgrade the quality to SD, which it sounds like you'd be happy with. My TP-Link AX1500 Wifi 6 router has that QoS feature. I don't use it. But, it's an inexpensive router and has been very stable and fast for the year I've used it. (I think you said you replaced your router. If it doesn't have QoS, you could return it and get this one.).
"People are often amazed at how much we’ve done with the number of engineers we’ve got." (Roku CEO Anthony Wood, Austin Statesman, Oct 4, 2019). "Amazed" is one way of putting it.