How to choose Xfinity internet speed

We're using Xfinity for internet and we're wondering if it would make sense to cut our costs by going to a slower speed.

Per our bill, we have the "X1 Starter Double Play" package which includes "Gigabit Internet".

It looks like these are the speeds and prices on offer:

Connect: $67/mo, 150Mbps download, 10Mbps upload
Connect More: $79/mo, 300Mbps/10Mbps
Fast: $89/mo, 500Mbps/10Mbps
Superfast: $99/mo, 800Mbps/20Mbps
Gigabit: $109/mo, 1,000Mbps/20Mbps
Gigabit Extra: $119/mo, 1,200Mbps/35Mbps

Our heaviest internet usages are probably 4K movie streaming, teleconferencing (school and work, sometimes a few of us at the same time), and games. I don't know details on the games because that's the kids.

On the upload side, I have published 10 Kindle and POD books and every time I upload an update, that's typically around 15MB-25MB, and the biggest one is about 60MB.

Any suggestions?

UPDATE:

It occurred to me that I'm using an old cable modem, and maybe I'm paying for faster speed than it can do? It's a Motorola SB6121, which was introduced in 2011, and which I've had since 2014. Motorola claims a download speed of 160Mbps. Does this really mean it downloads barely faster than Xfinity's cheapest "Connect" level?

I tried speedtest.net at a time when everyone else in the house was in bed, but without hooking up by ethernet as suggested for best results. The wifi router is an Asus RT-AX86U. I got:

134.84Mbps download
12.43Mbps upload

Does this really mean that we could downgrade to "Connect" with no download speed penalty and only a moderate upload penalty?