Break up module to make it easier to manage.

The patch is too big for me to review. ;)

You do as you wish, but I would only suggest that you wait until you port the module to Drupal 6 before breaking the module into different pages:

1) It's too late in the D5 cycle to introduce such drastic changes. There might have been some minor bugs, but the module worked well enough so far, so let's not chance introducing critical bugs in the process.

2) Major changes in the menu system in Drupal 6 make it more logical to wait until you are ready to port the module to D6 to introduce this kind of change. D6 has a new menu callback system that is designed to automatically include some .inc files according to the callback.
Check it out:
http://drupal.org/node/114774 Converting 5.x modules to 6.x
http://drupal.org/node/102338 Drupal menu system (Drupal 6.x)
and especially: http://drupal.org/node/146172 Page handler include files

Another reason why to wait until D6:

3) most people who use this module are completely new to Drupal. What they usually do is grab a copy of the latest release of Drupal and try to import phpbb into it. When phpbb2drupal for D5 was late in coming, people were not happy because they didn't want to get a copy of D4.7, import phpbb into that, and then upgrade the whole to D5.
D6 is just around the corner, now, and it won't be long before people will be requesting a D6 port of the module. Those who wanted to import phpbb to D5 have done it a long time ago. Those who are only now thinking of migrating have already their eye set on D6.
All this to say that another major D5 release is not worth the trouble. If I were you, I'd do the D6 port first, and fix bugs there and especially break up the module only in D6.