Your bit about "moving SQL" is utter nonsense. Fear mongering nonsense built on career educator programming advice. The only reason I'd separate it out isto objects as you describe is for multi-language support. In fact, most implementations that do the separation you suggest end up bloated and slow with function/method calls for nothing, wasting memory making complete copies of the result set instead of sanely iterating one row at a time, etc, etm.
That said, the markup is outdated trash... but a lot of that comes from it having NOT seen a real update in 15 years. I should know, I've been using it since ~2002. Even so it makes phpBB look like dinosaur dung.
I don't expect that to change despite their claims to the contrary with their dipshit use of jQuery and most web developers general lack of giving a shit about the quality of their work. Particularly since they've been promising a modernized 3.0 for over a decade now, but continue to slop out the same broken bloated HTML and CSS.
The camelCase thing? Originally SMF used both, and there are telltales of it as it follows an early PHP practice of underscores for sections/subsections and camelCase for names.
template_categoryRow() == categoryRow.template.php
This was used in the code, so it was used for the filenames. So long as you keep it straight, I'm not sure what the objection is. It's a convention I use myself and have used without worry for decades; even in other languages.
That said having come up on the Microcomputer side of the big iron divide and starting out with Wirth familiy programming languages (Pascal, Modula 2, ADA) I absolutely HATE case sensitive languages and filesystems.
The JavaScript thing I was fine with back when they used only 30k of the stuff as vanilla... but recent versions have really soured me since they were SUCKERED into the mental-huffing-midgetry of jQuery and then ended up using as much if not more code than they'd have had without it.
That's my biggest complaint, the jQuery bloat and incompetence that was only recently (five or six years ago) added. It did them no favors, apart from opening the door to developers who have no business working on the front-end to begin with.
That said it still gracefully degrades to work without JS just fine, so I'm not sure what your objection is in terms of "undesired conflicts" or whatever.
And really when SMF 2.0 first dropped well over a decade ago, 90% of the stuff we don't use JS for anymore we still had to use JS for.
I'm not a fan of "modules" beyond stuff like spam blocking. If the forum itself lacks the functionality to... well... be a forums, it is unlikely in my mind adding mods is a good idea. Particularly given how often they can neuter your upgrade path and open up security holes.
See Turdpress' SVN for proof enough of that. Prior to version 3.0 90% of its vulnerabilies were in the system itself. Since then 99% of the vulnerabilities are in mods/plugins created by random schmucks and installed by people who need to learn to keep it in their pants.