Believability of Speedtest websites

I'm having an issue with the network at my office. My main question is, how much stock do you put in websites that claim to tell you your Internet connection's speed?

For background, the network consists of 4 Windows 10 workstations, a windows 2012 server (responsible for DNS/DHCP), a Sonicwall TZ215 ( I know it's old, I've replaced it temporarily, it made no difference in solving my issue), and non-managed 8 port Netgear gigabit switch (tried replacing this too, it made no difference).

I'm running speed tests (mainly from speedtest.net, but also Google's speed test, and a few other sites) and think there is an issue based on their results. I'm paying for a 200 x 10 connection from Spectrum. Upload speed is almost always around 10 Mbps in every scenario, no matter if I'm plugged directly into the modem, or if I am running through a router/switch/etc.

For download, Speedtest.net consistently tests around 40-60 Mbps on all the office PCs (maybe 80 on a good day) (note: this is when going from the modem, through the Sonicwall, to the switch, then to a PC).

I've tried plugging directly into the modem, so there is nothing to slow it down, and I've 200 Mbps maybe 30-40% of the time, but also around 80 Mbps, and even as low as 30 Mbps. I called Spectrum, they said the modem appeared to be operating normally on their end, but they had a tech come out. He plugged into the modem, got 200 Mbps, and then took off without doing much else.

This has been happening since November, and the reason I tested the speed was because a virtual seminar I was on kept freezing, and my connection was slow on other sites, too. It has been okay over the last month or so, but I've haven't done anymore video conferences. This is an accounting office, and once our busy season hits in a few weeks, I don't want the network crawling once we're running at full capacity.

Thanks for reading.