Getting beyond the realms of my competence now but the amount of data here is totally huge and might well be beyond some free forum software. For example here I have nearly 1,000 incoming messages stored, and on most Joomla systems I've seen total mailbox size (incoming and outgoing) is limited to 100 messages.

Also at least one of the Volvo forums had a total loss of data due to (I think) a software problem and that would be most annoying: here there have been glitches but usually eventually stuff has finished up back where it ought to be.

I would personally say that a sub-forum system would be counter-productive. There is a significant merit in easy cross-fertilisation, and in may sub-forum systems things get posted several times in separate sub-forums so as to find thiere "audience". Also actually finding something is much harder if you have to burrow through sub-forums.