How can I measure actual internet usage speed?

I've been an xfinity customer for years on internet, and a couple of years ago they talked me into an 800 Mb/S plan with a bundle. Well, the time ran out and the cost skyrocketed back and they are not offering anything. So I want to drop my speed to save money.

I went to the xfinity store and had to ask about 5 times if they can tell me my actual speed usage. (They first answered that total data usage is viewable, then answered that I can run a speed test, etc. etc. etc...). I finally told them "Hey, you are the ones offering various speed levels, but I have no way of knowing my actual speed usage. Why can't you tell me that?" They said they don't have access to that data and they just go off of company guidelines (i.e. how many devices do you have, etc.). Well, I've been in sales and marketing long enough to know this is by design. Upsell everyone to speeds well beyond what they use, and make them think more is somehow faster/better, etc. 

If I do research, many people on the internet say 150 Mb/S is plenty for the entire house. Is it?

Has anyone actually measured their data usage under heavy load (multiple TVs, multiple people playing games, etc.)?

And is there any app or way to find out our household total speed usage (over time)? I'm using the xfinity xfi gateway/router. 

It appears the only solution I can think of is to add a new router behind the xfi (make xfi a bridge) then configure that router to run dd-wrt or some other platform, then install utilities (like YAMon) that can track speed. Seems like a lot of work just to get this information (that xfinity obviously has...).