Spurious update map reminders on Nuvi 154

Sadly not!! The spurious out-of-date map notice has returned (see photos) despite deleting the GarminDevice.xml file.

Is there any way I can reset the Nuvi to factory defaults and start again, to see if that fixes the problem?

You can try a master reset, yes.

OK, I did that, apparently successfully, according to the messages. However, I was expecting, when I reconnected to Express, to be told that the software and maps were out of date (as I had reverted to factory state, which was around 2015) - but it advised that I was fully up to date; which is weird and suggests that there is some other problem. However, I did get something I had not seen previously - a message asking if I wanted to reinstall the currently installed maps. So I accepted that and allowed it to do a full map update. I'll then have to wait again to see if the spurious message still occurs. If so, I'll try an alternate method of resetting the device, as the standard one does not seem to have worked as I would expect.

A master reset doesn't affect the maps

Aah. But since it is the maps that appear to be causing the problem, I was hoping to remove them completely and reinstall them from scratch. Also, my device is about 9 years old and, in that time, the operating system has been updated at least 3 or 4 times (once, I can recall one definitely, just last year). Yet, when I reconnected after the factory reset (which surely should have reverted to the original software), it said everything was up to date, so the factory reset does not appear to have changed anything at all. It doesn't seem likely that this will fix my problem, unfortunately, but time will tell!

It won't be the maps as they're up to date,  so likely an error in the software which should be sorted with the master reset. As you say time will tell.