I played this for hours last night, I'm going to need to block it :) Set myself a target of 2048 for nostalgic reasons, turns out it takes a long time (for me anyway).
You're right about the dictionary, actually the whole time I kept wondering about how annoying it must have been to choose a dictionary for this game. Even though not accidentally making non-obscure words without noticing is part of the challenge, accidentally making obscure words is annoying!
Maybe I just don't know enough words - but looking through my game log, I was annoyed by "cony", "smit", "huic", "yipe", "nome", "torii", "agon", "mairs", "imido" and "sial", some of which don't display a definition when you click them, but all of which appear in all the scrabble dictionaries referenced on the website you just linked. Meanwhile I was sad to discover vape is so far only in one scrabble dictionary :) And annoyed to discover "oxalic", which is also in all the dictionaries on that site, was not accepted.
I guess there's a spectrum between "advanced scrabble player level vocabulary" and "fun word game", because I imagine (and suspect you have probably had feedback along these lines) _not_ allowing a word which is obscure but still unambiguously used in the modern era would be worse UX overall - the sort that's more likely to make you rage-quit.
I can see why you'd try to get a bit of wordle-esque shareability out of the daily mode even though I like the classic mode more myself. But I think the tutorial popup isn't as comprehensive as it needs to be for someone's first game to be fun. The first time I clicked the link I did an abysmal job at the daily challenge, I think it wasn't obvious that swaps didn't need to be neighbouring like the given example. Something that might be better is to make an interactive tutorial for first-time visitors - come up with a 5x5 board that is quickly solved and demonstrates several strategies and then walk the player through clearing it. I also think the help popup being one click away would be useful.
I would also have liked the help popup to let me know that progress is saved if you close the page, I ended up checking in an incognito window because I had no time to keep playing but wanted to come back and try to reach the target I'd set myself another time!
Anyway - criticism and suggestions aside - well done, it is a fun game and concept!