Ended up upgrading mine today following a few guides. Took a few attempts but only once I realised I was misreading. Now on 6.1 and haven't lost any data! Haven't gone to 6.2 as the Jun Loader I use doesn't go that far but 6.1 is fine.
I initially went 5.0 to 5.2 last night, not sure that was necessary and coincidentally moved up to 6.1 about an hour ago.
I used this guide to go from 5.0 to 5.2 (
https://xpenology.com/forum/topic/4383-migration-dsm-50-to-52/) but some of the links are dead so you have to google for the files.