The Queen has sent them as her representative several times and from that they made friends with other young royal couples and got invites on their own merit.
Whilst they don't get the big splashy royal tours on behalf of the government very often, neither do/did Anne or Andrew (alleged favourite). Sophie has made many trips to places like Hyderabad and Kenya and the now virtual Sierra Leone trip with her patronages and obviously Edward would be in Canada if possible right now. I do wish these trips got more attention for various reasons. They did get an official tour in 2012 for the Diamond Jubilee to the West Indies and a few on other occasions.
I do think that Sophie is starting to get the "underrated, hardworking royal" articles that Anne has. Anne also went thought a very long who? phase in terms of visibility because she wasn't showy or interesting but I don't think that lessened her place with her parents. I also don't think that being the "forgotten, dependable ones" is such a bad thing at the moment, although it might feel like being the other brother with the Prodigal Son. Apart from anything else, they get to live their lives in peace as they see fit, unless they majorly screw up and they already know what that's like.
The situation with the patronages being given to the younger royals is unfortunate, although I can see why there was a push to go with the younger, high profile faces in theory, but that doesn't mean no one appreciates what they do with the ones they do have.
I'm taking the clothing situation with a pinch of salt but if at least partly true it wouldn't necessarily be unusual for that era especially with parents who grew up with rationing etc for the youngest in the family with two older brothers to live mostly in hand me downs, especially good quality ones.
I have no doubt rank does affect the family even in their private moments but they all have mixed feelings about The Firm but there hasn't been much to suggest that she bases her affections or appreciation on rank order, even though she was very busy for many years. Charles has talked about his difficulties with his parents and almost everyone BUT William has been mentioned as her "favourite grandchild" over the years.
https://royalcentral.co.uk/uk/a-look-back-at-the-wessex-royal-year-135537/
This look back at 2019 for the Wessexes suggests his parents do value their work, with the events they hosted, the Earldom Edward was given, the (not flashy) Patronage Sophie was handed and the trips they took. Many of them could not have happened without the acknowledgement of the Queen. They probably took up less of her time worrying about them because they didn't create an incident or terrible headlines though.