Personally I would go in 2023, before Fantasy Springs opens, or (as bswift suggested) go somewhere else now and then go to Tokyo Disney in a few years' time once Fantasy Springs has been open for a while.
I've just got back from TDR and (despite the reports of OCL reducing the capacity, dated tickets etc.) it was very crowded. This was probably in part due to the fact that the "Believe! Sea of Dreams" night-time show had just started a week or so earlier. However, I can only imagine how incredibly crowded it will be once Fantasy Springs opens - which I would assume would last for a year or two after it opens. I found the main impact for this was that the lines for counter service restaurants and food stalls/carts to be very, very long. This severely limited my options for dining, as I wasn't prepared to wait half an hour for every meal, snack and drink. I think this was the first time I've ever visited a Disney resort and not had a churro - but the lines were just too long. I really wanted a waffle from the Great American Waffle Company in
Disneyland but the wait was almost always 50-60 minutes. Most of the counter service places in DisneySea were ~30 minutes (except Yucatan Base Camp Grill, which seems to be quite unpopular so I ended up eating at several times...) I think I queued for half an hour for a chocolate thing and a drink in Arabian Coast, which is longer than I waited for most rides. Luckily there are vending machines peppered about, so you can at least get a drink without queuing.
I menion this in particular as your coeliac disease will, unfortunatly, already be a challenge at Tokyo Disney (compared to the other parks). Although I'm not coeliac I do have several close family members that are (although they weren't with me on this trip) so I have some idea of its impacts. When the parks are very crowded, your dining options will be more limited (unless you're willing to wait for a long time) and it will be slightly more challenging. In addition, cast members' English is not generally quite basic (by which I mean it's easy enough to order food from the menu but making changes/adjustments or asking ingredients is almost certainly beyond their English abilities and you'll have to use pre-prepared translations, google translations etc. and they'll probably have to call another cast member). Weirdly in my three weeks in Japan, I found the language barrier to be larger at Disney than anywhere else - at Guest Services they had to get another cast member to help me, at the TDL Hotel when I called to say "I have laundry to be collected" they had to patch in an external translator etc. etc.
Sorry to be a bit of a downer, but if it were me in these circumstances I would be doing everything I could to go when it's least crowded, which probably means going in 2023 before Fantasy Springs opens. DisneySea is still an amazing park with plenty to see and do so you won't feel like you're missing out.
The flipside of this is that elsewhere in Japan, at nice hotels, I'm sure they can accommodate your coeliac disease, especially if you can email them in advance to explain. I stayed at a lovely ryokan on Miyajima and emailed them about my shellfish intolerance and they had no issues accommodating it.
