My initial thoughts are that there's prob too many sub-forums and it would be easy for posts to get lost. Also that the SDGs are both broad and vague and not terribly well-defined.
However, they are also a logical way to order posts (and it may be just me that finds the SDGs rather unwieldy). I also would not be totally sure what other structures you would have. It may depend on what the userbase does, but if it's almost entirely health, then lots of the sub-forums would rarely/never get used. You could use geography for the sub-forums e.g. users interested in issues in Africa, Asia, global etc etc. But that may not be a good fit for many poeple too. Or you have have natural science/social science/clinical/laboratory or basic science etc etc. Which might work, but too many questions may simply not fit into any of the pre-designed boxes.
In essence, Andy - I dunno, really. Designing a categorising system anywhere is rather tricky! SDGs is probably fine, but is there a way to have a forum with 'all' posts listed? (Or at least, all posts from the last month or something like that?). And a good search faciity too. I see there is an advanced search, but prob a couple of additional filters could be added like 'search between these dates/search in the last 2 weeks etc'