Hi. I think an upgrade sounds like a good idea because Crittertrails are too small to have normal behaviours in, even if you have 3 joined together (one cage needs a good sized floor area and a deep enough base for Substrate). Also, as you say, the tubes are risky.
I wouldn't do the Billy extension though as it is too narrow for a Syrian (ie not deep enough widthways).
Not sure what the dimensions of your 120 gallon tank are, but as long as it is at least 40 to 45 cm (18 to 20") width (the short side not the long side) then I would suggest trying that again - but set up so one end is a deep digging area - this will effectively reduce the size of it inside - at least initially.
Having said that, it might be that your hammy is just used to a barred cage and not a tank - but he would adjust.
Transferring them carefully can help the tank to work for you. So for example you could set the tank up with lots of substrate (the more you have, the less you need to do clean outs and can just spot clean - for months really - especially if you provide a litter tray in the area the hammy chooses).
So put lots of substrate in - at least 6" - and then deeper at one end to say 10". To create a cosier main living area in the rest of the tank. (I'm assuming the tank is about 100cm or more long?).
Then put one of the Crittertrails inside the tank, without cleaning anything - ideally the one he usually nests in. And leave the door open. Then set the rest of the tank up with wheel not too far away and his old toys, food bowl, water bottle etc.
With a larger area too you might need more/larger toys to fill it a bit so there isn't too much open space (which can freak them a bit after moving to a bigger cage).
Pringles tubes are a good size. I also like the large cork log tunnels you can get in reptile shops (or on Amazon) - they usually need a bit of a wash/brush before use and I tend to freeze them for a week to kill anything that might be living in them!
The make a nice big dark tunnel in a large cage, and also something to climb over with an interesting texture - plus they are light and don't sink into the substrate.
Ideally you would want a large-ish house for nesting as well (a shoebox is a good size - cut a hole for a door and cut the base out and keep the lid as a lift-off roof so you can check inside without his nest falling apart). But you could wait to add the house later, until he's settled in (as the crittertrail will be in there).
A platform is quite important so they don't feel too exposed and have something to sit under and it's a good place for a food bowl, sand bath or any heavy/ceramic toys that could be tunneled under in the substrate. Just a piece of untreated pine wood and stick some dowels on for legs is fine. Or some people use Ikea Knuff magazine racks to make a platform/hide. (See link below) which might be easier if you don't like diy much.
So set it up so there isn't too much open space, a place to sit under/hide and plenty of hidey floor toys like tubes/tunnels - larger ones - and a cardboard tissue box or something can make a hide to fill some space too. Kitchen roll inner tubes are ok if you slit them down one side, and cardboard egg boxes with a hole cut in one end.
Then he can continue to nest in the crittertrail inside the tank, and gradually explore out of the cage. Actually I would put a house in at the start, as he may move out of the crittertrail and into the house and then you can remove the crittertrail after a week or so. Scatter some food round the cage, add an interesting nice chew or two and hide the odd treat around the cage. That will get him distracted and exploring.
Then just leave him for two weeks - no cleaning - just put food out. If he wants to come out that's fine, but maybe not for the first 3 days unless he's showing he wants to come out.
It can take a good two weeks for them to adjust to an upgrade. I did it with our last Syrian and he looked scared and shaky and we so nearly moved him back into his old cage (but couldn't as he was getting stuck in tubes), but stuck it out. And suddenly by day 10 he was happy as larry and loving it and looking really happy! Running about - leaping out of his wheel and flopping on the substrate (plenty of space).
Not sure what size wheel you've got but I'd invest in an 11" or 12" one as they run better in a large wheel and it can never be too small for them then. If you want to fill the space as well then rather than buying lots of extra toys you could maybe get a 12" flying saucer as well as a wheel so something else fun.
He will really appreciate the extra space and ability to have places to go and things to do, if you set it up carefully and give him time to settle. Leaving the Crittertrail in the tank for at least a week. He'll probably abandon it within a week if there's an interesting bigger dark new house to nest in.
And put a big pile of torn up strips of plain white toilet paper somewhere in the tank so he can build a nice big nest and forage for the nesting material.
I would change him over now if he's having issues with the tubes - even if you don't have a platform ready yet - you can add that later and just have plenty of hidey places and tubes for now.
What are the dimensions of the 120 gallon tank? The Billy is only just big enough for a dwarf hamster really and some people even then think it's a bit narrow.
Forgot to mention - when you've filled the tank with substrate, keep his old substrate from the other two crittertrails (leave the one he nests in as it is, in tact) and sprinkle it on top of the new substrate in the tank so it smells familiar. And move everything over without cleaning any toys/wheel etc so it smells familiar. It's tempting to clean everything at such times, but better not to - it'll be fine for a couple of weeks.
After a week you could spot clean his toilet area (ie any area pee'd on) by taking a handful or two of substrate out, adding a bit new and mixing it in - you could then put a litter tray (any kind of dish with chinchilla bathing sand in) in that area, and he should use it. Then you just need to empty and clean out the litter tray every 5 days or so and the rest of the cage should stay dry
But don't clean anything or move anything around for at least two weeks once he is in the tank