Been having the same issue with my six series. Going into settings and restarting the TV seems to fix the issue for a bit, sometimes a few days, sometimes a few hours, but eventually it gets out of sync again.
A bigger issue has popped up since the upgrade to the new 10.0 OS, and that's the audio just goes away sometimes. Normally it's when switching apps, but sometimes I'll be in the Youtube app, and when I switch from one video to another the audio is just gone. The easiest way to fix it is to get up, unplug the HDMI from the earc port on the back of the TV and plug it back in. And the audio sync issue is still there.
Similar issue pops up every few days where Dolby Vision just won't work until I do a restart in the settings menu. I was in a similar position as you, choosing between the TCL Roku and a few other TVs, and unless Roku (or TCL) can get this figured out, I'll probably avoid Roku and TCL TVs in the future.
The TCL Alto 9+ soundbar (TCL's flagship soundbar) was even worse. I can't adequately express in words what a POS that thing was. It contributed to the audio sync issues and the new firmware (2.9) which was supposed to fix all the problems ended up completely breaking the soundbar instead and making it unusable. It's now sitting in my garage gathering dust while I wait for TCL to figure out what to do. They even sent me a new soundbar but that one had the 2.9 firmware on it too so it was also useless.
But even with their soundbar out of the equation and replaced by a much better LG soundbar, the TV still has audio sync problems. It's most noticeable in the built-in Netflix app and ironically, in Roku's own Roku channel. I've had to give up on using the built-in Netflix app and have been using an Apple TV 4K, which I only bought because Roku no longer carries the Spectrum app, for most of my streaming apps.