G&P - a few suggestions to try
The Garmin-supplied 1-50k Ordnance Survey OS) map file that came with my old GPSMAP 62 is 3.2Gb. According to the
GPSrChive the Etrex 20x only has 3.7Gb available. How big is your Open Source (OSM) map? Should it fit?
What happens if you delete the Open Source (OSM) map from the device internal storage and then re-try the card with the Ordnance Survey (OS) mapping? Does the OS map work?
If you put the card into a card reader and open BaseCamp, can you see the OS mapping?
This might help determine if the card or the OS map file(s) are corrupt.
I wonder if the device is freezing because it is trying to load both the vector and raster maps at once? Sussamb will correct me if I am wrong, but I think when you put a card with a raster map into a device and turn it on, by default that map is enabled in Map Setup. It is worth taking the OS card out, turn on the device and turn off the OSM vector map so you have a blank screen. Turn off, put the card back in, turn back on and see if the OS map is enabled and visible. If it is, don’t try and enable both the OSM vector map and the OS raster map at the same time.
Last suggestion is in future to use a bigger card (GPSrChive say the device should take cards up to 32Gb) and put both OS and OSM map files onto it. It might be slightly slower, but you will have plenty of headroom for other maps. You could also try making some CustomMaps in Google Earth KMZ format of the areas you are walking in to see how much better raster imaging can be compared to Garmin's offering. There is a tutorial here:
https://support.garmin.com/en-GB/?faq=cVuMqGHWaM7wTFWMkPNLN9 and JPG map images up to A3 size are available to download from the DEFRA Magic map site.
Good luck!
BTW I believe the reason the Garmin OS raster maps are so poor is that they downsampled the jpg resolution so that they could minimise the file size and map loading time on older devices.