Hi there. That must have been so hard to reduce your pets, with your OH's asthma - but I know what it's like - I'm badly allergic to dogs - and most animals. I too am not allergic to hamsters - yay! Lovely little fluffballs too. Apologies for preaching to the converted. Yes I can understand a smaller environment without fall risks could be good for a robo who needs a lot of cover.
As to the bar spacing in the Imac Fantasy - this thread reckons it's too wide for Chinese and dwarf hamsters generally. It's from 2008 and the Imac Fantasy seemed quite popular back then - I think over time though it's been accepted it's a bit cramped for a Syrian - and tube size can be an issue for a Syrian.
Imac Fantasy - Cage Review
I can see how a Duna Multy could seem a bit empty when the Imac is set up to go, a bit like a Hamster Heaven, but I've seen some good dwarf set ups in them and cork tunnels are very good for cover and something to climb over.
Since 2008 the focus went onto minimum cage size and is now more onto "enrichment" being the important thing. Enrichment being depth of substrate - enough for digging and making burrows and tunnels, variety of textures, levels, variety of toys and hidey places.
I'm not keen on the Duna Fun for dwarf hammies - I think there are some fall risks and the odd gap between shelves and ladders. But I can quite understand why you like cages that come ready set up! I'm not great at thinking up set ups for an empty cage myself and hated it at first, but quite enjoy it now I've found things our hamster really likes! There's the cost as well of buying "extras" for a cage.
I think the key with an open space cage for robos is to give lots of cover toys on top of the substrate - cardboard boxes, tubes, tunnels, shelves and so on - if the substrate is deep enough (at least 15cm - ideally 20 to 30cm) they can keep undercover by tunneling. A subterranean house can be good too (part buried in the substrate - you can have a tube leading down to it) which mimics an underground burrow and is easier to clean out! Labyrinth houses also seem popular as they're dark inside and have different rooms for nesting, hoarding and so on. Someone on here made their own labyrinth house out of cardboard boxes, but the Trixie Leif house is popular and Rodipet make great labyrinth and other houses. A bit expensive though.
Personally I love the hyacinth tunnels. They can chew them safely (no sharp spikey bits) and the holes in the side seem very popular. Our Syrian isn't interested in toys generally - unless it's round and cosy! But he loves the hyacinth tunnels and weaving in and out of the holes, chewing it and sometimes just napping in it.
https://www.amazon.co.uk/d/Small-Ani...acinth+tunnels
Having said all that, if you prefer the plastic items for cleaning, and really aren't into setting up cages (which I can understand!) and prefer something that comes with the items so the cage doesn't need much adding to (ie like the Hamster Heaven and Imac Fantasy) then it is a bit limited in cages with small enough bar spacing.
How do you feel about wood cages (and plastikoting). A few people have had this one - it has a large built in shelf which gives lots of cover. It comes flatpack and when you put it together you can swap round the door and front panel so the door is at the top - and the base can be filled quite deep with substrate, so cushions any falls and hard toys can go under the shelf. It might not even need plastikoting as it has a white melamine floor. Escape proof - a couple of shelves for cover and comes complete with houses and ramps
https://www.amazon.co.uk/dp/B01HHIV2YA?psc=1
Zooplus sell a smaller type one with a shelf under but it costs a bit more
Loft Wooden Cage | Free P&P on orders £29+ at zooplus!
A member on here makes fantastic ones as well - here's the website - she also sells them on Etsy. They come in a range of sizes and don't need plastikoting (already waterproofed with pet-safe coating).
Hamster Homes Shop - Homes & Accessories for Small Pets