1080p Zoom HD only calls for 3.8Mbps. A 20Mbps up connection should be just fine if it's truly 20Mbps up.
I have a couple coworkers on StarLink who can only get 20Mbps up on a good day. Zoom is still fine.
I agree, though "if" is kinda key. And the number of concurrent users in your household. It can be easy to accidentally saturate your uplink if you have multiple users in your family and you aren't coordinating. Or at least that's been my experience.
Switched to city fiber as soon as it was available and that's been a blessing.
You probably got a different modem, with more channels, that opened up more actual bandwidth for you.
Outside the US coax based internet is rarer (its fibre or copper) so there isn’t a lot of good modem/routers.
So if you can’t get fibre, get an Eero or similar and at least replace your router.
When people replace modems they often get an upgraded one that can support more/better channels or DOCSIS versions. These new channels usually are much less contended (as only a tiny fraction of people have them at first), so it seems like a huge improvement, then as bandwidth demand grows and more people get new modems which can support the additional channel it begins to degrade.
However, I would say that with DOCSIS3.1 and DOCSIS4.0 you are starting to get to a point where there is so much more capacity available and bandwidth growth has slowed down so much you are starting to see a lot less capacity problems IMO.
needless to say, they're often overlooked.