Nohal is of course, as always, right but I think I understand your intentions. These has been discussed years ago but then the majority of the community didn't want timeout for depth
My feelings are it's better to show a question mark than the last value in Dashboard if the sounder fails. So I made this for my own use. If you wish you can try my attached Dashboard. (Pls delete the .pdf extension) But beware, there are some other minor adaptions for my dated GPS-navigator, Garmin
120, and the as well dated Simrad
IS15 system but I don't think these will bother you.
The timeout function is using the GPS
timeout timer as the heading instruments in Dashboard are.
The attached Dashboard is based on recent OCPN Github code.