Don’t get me wrong a CHE is a very good way of heating an enclosure, just from my personal experience and experiences of the Retic owners I know, I wouldn’t recommend them. Of course people have housed them using the CHE as their heat source and had no issues with accidents, I myself just wouldn’t risk it. Using one with a cage I think is a good idea, as long as the cage doesn’t get hot to the touch.
Given she’s a year old, and 3.5ft, she doesn’t need the largest enclosure right now. Sure the extra space is nice for them, but as you said she’s not very active right now, and she’s in a reasonably decent sized enclosure for her size at the moment. I would personally wait till she’s about 4-4.5ft before you upgraded her.
That’s good that she has a tame temperament. That will allow you to go as large as you want, Retics that are a little more defensive don’t like the extra room or height in most scenarios.
If it was my Retic, I would wait until she grew another foot or so, especially given she’s not very active.
When it’s time to upgrade her, and you could do it now, definitely up to you and it won’t hurt her at all to have more space, I would go with a 5ft wide 3ft tall 2ft deep. You can go a little larger or smaller it’s really up to you.
Only thing to keep in mind for the larger enclosures is the humidity is harder to keep up around the levels Retics should be kept at, and it’s also harder to heat as you know. And cleaning can turn into a real pain, because I’m sure you experienced they pee pretty much everyday and unless you spot clean( I don’t, I clean everything whenever they pee or poop) it can be a choir everyday to sanitize everything.
Again you have a lot of options and can’t go wrong with upgrading her or leaving her in what she’s in now.
The biggest thing I think Id ask myself tho, especially because it sounds like she’s not as active as some Retics, do you want a lager enclosure or does she need a larger enclosure? She will do just fine in what she is in now, and you don’t want to invest in a large enclosure unless your snake will use the extra space. One of my Retics sounds like your girl, she never moves or explores. Another one of my Retics never stops moving. For the active one I’m going to give her a very large enclosure because she will use it, and for the other one I’ll probably go half the size of the active ones enclosure because she could care less if she has extra room.
And if you’re worried about her getting exercise or enrichment, take her out everyday. That’s the best exercise and stimulation they can get.