If you want basic interactivity, HTMX is fine.
If you want something more (like sortable) I'd reach for lit instead of adding the Sortable.js dependency, something the HTMX docs suggest ( https://htmx.org/examples/sortable/ )
And yes there are also some scenarios that you want to control the whole routing and rendering stuff, then go for Solid, React, Angular, Vue, whatever.
There is no golden rule to all of this but finding out what makes sense is a fun exercise.
But interactivity in largely content-driven sites doesn't necessarily preclude HTMX. You can do a bunch of fancy interactive stuff with HTMX and some minimal JS. Drag and drop sort is a tiny drop-in JS snippet. Skeleton loaders and spinners are supported out of the box in HTMX as is infinite scroll as others pointed out.
I'm not saying Next.js, Solid Start, Remix, Nuxt or some other hybrid server and client rendered model doesn't work for forum software. Or that even client only rendering can be a valid approach if you accept the trade-offs (no curl-ability). People should use whatever they feel productive building with.
But if your backend is already a non-JavaScript server rendered framework like Django then HTMX makes a ton of sense to add some interactivity.
Otherwise, I'd recommend HMTX even if you have a super flexible JS backend.
That said, this is all baseless thinking without seeing any requirements and as I said before, trying to choose the right abstraction is the hard but fun part.
"Fun" as in you're years into a project before you fully accept that you made the bad call.
Everything served under one domain doesn’t have to use one technology.
Use backend rendering when you can, use HTMX where you need a little bit more, use React on the actual bits where it matters.
As a side-benefit, the React bits might end up less complicated as React doesn’t need to handle every single use case.
Good old server side rendering is simple in Rich Hickey's "Simple Made Easy" way. You can build more complex logic on top of a standard server rendered model, but the base model is mostly dead simple to reason about. I like that. I've built web "apps" in AngularJS, React, Reagent (ClojureScript) and Next. So I'm not coming at this from a grumpy "backender" position. I'm coming at this from having done lots of frontend stuff and finding the frameworks becoming increasingly complex beneath the surface. It feels like they are targeting the "easy" in Rich Hickeys parlance in the sense that you don't have to write a lot of code to make something fancy. But reasoning about the behavior and what actually happens in the framework when something doesn't work is increasingly difficult.
Edit: And maybe this is a "skill issue". If you just learn the framework and library well enough you'll get by much better. But I tend to not want to bind myself to any single framework too much. So simple frameworks that don't churn too much is better for me.