Say you're having a conversation with someone broadly in the same line of work as you whom you don't know very well, in an only mildly informal setting—like, near the snack table after a talk you both attended or something. One of you has asked the other about something technical and the other is in the middle of explaining. You admire some of their work, and want to mention this as a brief aside amidst the "cut-and-dry" technical talk. (American?) English speakers will often introduce such statements with a brief opening phrase that sets it apart from the "serious" material: "By the way, I just wanted to say, I really enjoyed your massive treatise on duck courtship rituals from 2012," or, "Oh, just as a side note, I thought your kazoo madrigal was really great, it was so inventive!" Does something similar happen in Japanese, and if so, how does it play out?
The best guess I have is using「お伝えしたいことが……」, which seems to capture something akin to "I wanted to say" with the right combination of formal and conversational(?) But I feel like there are probably other approaches that would also work (provided that that phrase even does work which I'm obviously not sure of). There's also 「言っておきたいことがあります」and variants but of course those seem more along the lines of "There is something I want to tell you" which is maybe not quite right for this case (too much like you're preparing to admit something, maybe).
There's also terser phrases like「ところで」or「ときに」.「ところで」seems maybe a bit too terse and informal…? 「ときに」seems maybe a bit closer but also kind of overly casual(?), and also it seems to carry stronger connotations of "sometimes" or "now and again" or the like that might kind of clash weirdly with this usage(?) Maybe there are ways to work these sorts of words into larger constructions that would have the right nuance?「ところで、お伝えしたいことが……」perhaps…? I'm a little worried「ところで」is just slightly too familiar but I'm not positive—maybe it would come down to personal style or the particulars of the conversation?
There's also much stiffer phrases like「注記として」or「補足として」but it seems kind of weird to use those outside of like a formal written document or at best a prepared lecture or something. Again, though, maybe I'm wrong.