Firstly, I'm so sorry about your Nan. My condolences. I hope you're doing okay - or as okay as you can be. It's especially hard to lose someone at this time of year and my heart goes out to you.
As for thank-you gifts, what about a little basket or box filled with a combo of some of the following:
Little things to decorate the office/nurses station/hallways - like potted succulents (you can get really nice looking fake ones from poundland, and real ones for not much more at IKEA), any kind of artwork - especially framed prints, or ones mounted or on canvas
Pretty stationary: (think Kikki.k - in style, not price point!) nice pens and pencils, a rose gold stapler, fun paperclips, pen holders
Face masks: they can never have enough, and even if they are mandated to use disposable ones at work it's always nice to have extra or to take home. You can find really well made reusable/washable ones on etsy, with a pocket for a carbon filter (which may make them work-approved) and the carbon filters can be found easily and cheaply too, and both can usually be found at a discount when buying multiples
Flowers: always a classic and a great gift. You could also buy a nice vase, and pick out some quality fake flowers (you can buy ones that come with scent - you spray it on the flowers every few months so they look & smell like the real thing) - that way they'll have a totally customized and long lasting bouquet. On that thought, maybe a diffuser for the station? Lots of pretty and unique designs, just pick a scent that's fairly neutral - like White Linen
Lanyards: again, Etsy is a great resource. Or you could get them badges or pins to put on theirs - anything you like, whether they're cute ones or ones that express thanks or a saying or quote you like - that way they'll always have a little token of appreciation with them
Coffee vouchers: giftcards for the closest local chain (Starbucks, Costa etc) with a small amount on each - that way they can treat themselves to a fancy coffee or a brownie or something while on break or after a hectic shift
And of course a card, expressing your thanks - it'll be hung up and treasured.
These are just some off the top of my head, but I do hope they're helpful and give you some ideas to go off! Let me know what you end up going with, or if you end up stuck for ideas - window shopping and present buying are two of my favourite things so I'd be happy to help! And interested to hear how it turns out, as I think it's a lovely thing to do.
I worked in a care home a few years ago and while they're all different to some degree (in terms of what's allowed/rules) I think these things would all be well received. I can guarantee you, whatever you gift them, they will truly appreciate and love the gesture. You seem like a thoughtful person, and your sincere thanks will mean the world to them - everything else is just wonderful extras :)