My Venu SQ shows the following message permanently: 'Memory low. Upload to Garmin Connect'

You have to connect the watch to your computer and manually clear out the old activities to free up space.  It's extremely annoying you can't do it from the app.

 Are you seeing current information when you open up Connect?  Do you see you latest activities?  If so, you are syncing, it’s just that your watch’s memory is full.  If you are seeing current activities in Connect then you can remove old activity files *.fit from the watch by connecting it with the charging cable to a pc and removing old .fit files under the activity folder.  It normally should do this on it’s own but once this has happened to me before also.

 You can also remove some watch faces and/or content if you have downloaded those from the Connect IQ store to free up memory.

You can do it from the watch settings. There is an option to delete all activities. They have now all gone from watch but remain on the app. Seems to have resolved this issue for me without need to connect to a computer

You can do it from the watch settings. There is an option to delete all activities. They have now all gone from watch but remain on the app. Seems to have resolved this issue for me without need to connect to a computer

I saw that measge when there was a pending update on the watch which I needed to execute first. Are you sure there is no update file synced to it ?