I assume you enjoyed your Seabourn experience, given your past and future sailings with them? My apologies, and please ignore, if this is diverging to far from the original topic.
You're right about the ultra luxury vs luxury distinction, which I'd forgotten about...I spent some time a couple of months ago on the Cruise Critic forum (just reading other people's posts) and learned a bit about that. I also learned about the "premium" lines (such as Viking and maybe Oceania?? I might be misremembering on that one) that some people lump in with the luxury ones (incorrectly, according to CC...lots of strong opinions on that over there). I was initially drawn to Viking because I heard about the educational presentations and experts onboard (my husband and I love to learn, especially about animals and nature), but I read too many negative reviews of the food situation for vegetarians (both DH and I) to want to sail with that line. With Disney, we know we'll be covered with decent to excellent vegetarian food, and after a poor experience on RCL, that's a big concern for us.
Yes we've loved our experiences with them. I've told people on the DIS before about the price difference as well lol meaning what you said about it being close in cost depending on the trip. With Seabourn tipping is neither required nor expected and there are no gratuities charged to your room each night, you do not see that 18% gratuity charge from
DCL for things (spa stuff excluded like normal), you want a specialty coffee you just order it, you want an adult beverage you just order it.
While I never finished my trip report despite my promise (ugh I'm bad at that) if you have signatures turned on you can click on my thread for my incomplete but certainly still decent trip report from my first cruise with them in 2023 in the Adriatic sea (Encore) which would give you an idea on the rooms and ships, our 2nd cruise was March/April 2025 in the western Med (Ovation sister ship to Encore). Our upcoming ones are on Quest but they are for end of 2027 (Caribbean) and mid-2028 (Baltics)
Encore and Ovation are 100% all verandah and price point are better than Quest in our experience because Quest has the lowest category of Oceanview.
Alaska seems to be the equalizer in terms of destinations. Seabourn does provide you as yours to take home with you a Helly Hansen jacket for the Alaska sailings (if it's only Alaska as its destination not if you do a World Cruise or a transpacific sailing). For presentations on board Seabourn does that as well and at least on our western Med cruise if you missed the presentation you could watch it on the tv later on, I watched a fantastic presentation on the Moors, Gibraltar and more (as we were going through the Iberian Peninsula) as well as another one on Napoleon (as a portion of our cruise was in those areas/time period). Alaska is going to be more wildlife oriented so you may have more talks on that.
As for the food it is IMO very good, on the vegetarian aspect I felt they stepped up quite a lot in terms of that on our 2025 cruise. The menus listed in a more defined way gluten free (GF), vegetarian (plant icon), vegan (V), no sugar added (NS), and contains nuts (CN).
Here's a sample from one of the nights of the menu from our 2025 cruise, I know it's really hard to get a good pic so you may not see this as super helpfully but hopefully it gives you an idea. The Restaurant (which is the MDR) menu is also available during dinner hours as in-suite dining and can be served course by course if you wanted.
Don't confuse Earth & Ocean for your place you just plop down in swimwear, that is during the day. In the evening it's just like the other restaurants but more casual in atmosphere given it's outside.
Solis is their reservation only specialty dining restaurant but there is no additional charge for the restaurant. In Encore and Ovation there is a small but dedicated Sushi place that during lunch serves bento (cooked) meals and raw at dinner, we only liked cooked so we go there during lunch....the sake (certain ones included) can be dangerous

. For Quest Sushi is served at The Club rather than being a stand alone restaurant.
There's a lot of overlap on the ultra luxury passengers as far as the more tenured ones have sailed with Regent, Silversea, Seabourn and most recently Explora. Many of them have done Windstar, Azamara or Oceania as well.
My best suggestion is to maybe join a Seabourn FB page (try to find one that isn't just a
travel agent sponsored one as you're going to get too much of that stuff instead of people's actual experiences), you could absolutely ask your question about the vegetarian options also ask about Alaska specific cruises and you can also get an idea of the cruises for Alaska by looking at people's posts.
As a comparison for the ship and experiences it's very difficult to compare DCL because DCL is not at all the same target demographic nor style. Even skewing higher income people are sailing each line for completely different reasons.
You mentioned dress code yes there is one for the whole ship after 6pm it's elegant casual though vaguely a few years back "elegant jeans" were added as acceptable but no one really knows what that is lol. Then there's formal night which only applies to The Restaurant whereas the rest of the ship stays elegant casual. Certain regional areas become more casual due to the climate. Antarctica for example doesn't have formal night on the Expedition ships. I'm not sure how much people adhere to the dress code on the Alaska cruises but that is for sure something you could ask on the FB pages. Seabourn is usually referred to as the country club ambience for dress codes.