Following Wirecutter's suggestion, I got a ProForm 505 treadmill. Like Wirecutter said, it's cheap, a bit janky, but does the job adequately for a "non-pro" like me.

When unpacking it and setting it up, there were multiple notices everywhere: On the packaging, as a separate note in the packaging, in the manual, on the treadmill itself. Those notes all said that the treadmill is "locked" and you need "online activation" to unlock it.

I was getting very nervous, since I thought I bought something that does not need online activation.

However I think it was also Wirecutter that mentioned that you can just press the iFit button for longer than 15 seconds--or was it 30 seconds?--and it's "unlocked". I did that once and it worked ever since, never needed to do anything online, or connect it via Bluetooth, WiFi or anything else.