With only 3 or 4 streams, 300Mbps down is more than enough for your usage. Netflix 4k streams are about 20Mbps each, Apple TV 4k streams are 40Mbps and Amazon Prime UHD streaming maxes out at around 30Mbps. So at most, you're using around 150Mbps at once with 4 streams going. You will not notice any difference with a higher speed modem. You should downgrade to the 300 or 400Mbps tier and save a few bucks a month. If you have issues with lag or signal anywhere in the house, getting a better router would be a better choice.
If it's all working just fine, I say don't change a thing if you can afford to. An "upgrade" can make things worse.
I don't pay for unlimited. But I've almost come close to going over that 1.2 TB mark. Internet I'm paying $60/month along with the home phone $30/month. I got a package deal from Comcast after I basically called in and said I was going to cancel everything if they didn't give me a better deal. Along with Youtube TV, its all $145/month The bill was like $250 before that.
