I've been trying to use assistive touch and it seems to be very good,

I tried the one with the most downloads and it worked fine on my Optimus 2. I did encounter a barrage of ads while setting it up (custom menus), but in day to day use I did not encounter ads.

I am now trying the one marked as ‘New’ on my LEM 12Pro and while from the same developer is a little different in the way you customize. Ads are rampant during setup but using actual button day to day, I’ve not run into ads.

While both of these are from the same developer, the first one is subscription based and the second one (New) is a one time fee. Both have been updated in August 2021. I think continued development is important and I do believe in supporting developers. Neither has presented any issues to date on either watch.

I own Smart Touch Pro and it has been pretty solid operationally on both watches. I do find the print to be small for my old eyes and the favorites menu doesn’t scroll and therefore I’m limited to 6 favorites. Maybe the screen size limits the number of favorites. The biggest concern for me is that it is no longer supported.

Thanks for these informations , I found one which seems to work correctly but the only drawback is I have to activate it each and every I awake the watch …

Sounds to me Battery Optimization needs to be turned off.

This seems to be a feature of Sidebar 2:

The Notification
It prevents Android from killing Sidebar. Without it, sidebar could not run forever in the foreground and would eventually be closed.

Also, I noticed this app hasn’t been updated for 8 years. I wonder if Android’s security evolution is at play with your issue.

No I checked, battery optimisation is not activated, may be it is a limitation of the lem15 stock firmware ? It works fine on a samsung tablet