I do know people who have had their Visas revoked after going back and forwards between Canada / US / Mexico. It was a bit complicated and not easy to explain here, my point is that it can hahappen.
Be very wary of unofficial US.GOV websites offering visa advice. I've just seen one where it advises that an extension is granted in increments of 6 months.
We have copies of our I539 extension application but I think they're at home and I'm certain the maximum extension was one period of up to 6 months.
Applying for an extension is stressful even if you have a good reason. Due to covid during our application, it took over 6 months from date of application to it being granted. During the extension period, you are technically an overstayer.
If the application is refused, you will lose your Visa entitlement and be subject to deportation without right to appeal.
I can't say how long application to extend takes now, or what happens if the application is denied whilst your visa entry is still valid.
To be perfectly honest, if it was me, I would forget any extension, and visit for the granted period, then go home for a couple of months before returning.
We also read somewhere that you cannot reset the I94 (entry and exit record) without leaving North America. That includes Canada and Mexico, but after 6 months in the US, we visited Canada with the intention of going right over to Nova Scotia.
After 3 months we found Canada extremely expensive and decided to try and re enter the US. We had to have a secondary interview with a border official where we explained our case but that we understood we had no entitlement to re-entry. We showed our return flight booking and they gracesiously let us back in.