The rig actually seems really solidly built and in good nick but it is getting a bit long in the tooth now for Dyneema if what you're saying about UV degraded strength is on point
Basically 95% of 'fibre rigging' we see in NZ is made by 'kiwi lads given it a go', it is nothing like what you'd get if you went to say Soft Rig, a big manufacturer to race yachts up to Superboats. Some of the NZ stuff has worked OK to very well, like 1paulg for example, his is a ideal boat (multi) that had a builder who was both fussy and smart (it is a very nicely build ride) so was set up well for fibre, which looked to have been made by someone with good skills. There are a reasonable number around with 'NZ way' fiber working very well, but there is also a lot who have tried to find it had been installed with too higher ratio of Mr Cock-Up.
If I had a multi I'd be using fibre and be fine with using the NZ version, history has shown it works well at a fair price if done properly. But I have a mono, the lastest DM range of no creep genuine Dyneema at work, minimal price concerns, the knowledge to make safe shrouds and I do like a good bit of wank factor. I also have access to the real McCoy from Soft Rig at cost prices but I am paying Yacht Spars to make me a new set using wire because I want stable and reliable without spending near moonbeams times the cost of wire.
UV degradation depends on many things, as does strength retention. To say X will do Y in Z time frame is a bit tricky due to all the variables.
Do a visual check for cut's abrasion and the like. If you find anything you think maybe dodgy the chances are it probably is so suss it deeper quick. If all looks good then fingers crossed.
If you are worried you can take one shroud off and get it load tested. If it fails then you know, if it's OK then you'll know. Pick one that gets a representative amount of everything.
With over braiding or sleaving, it's best to have it body hugging like lycra on a hot 30yo rather than granny stockings slumped down around her ankles. If it's body hugging it will be obvious if issues are arising, grannys stockings hide spooky very well.