Good thinking on the 45, 90 and 180 indexing and and 1:1 drive.

If I was going to cut gears with CNC I would use a conventional form gear cutter and just index the blank under CNC as usual.

In my opinion having idlers that pull in close to one another does mean you need to have a longer belt to give you a gap between the servo gear and the 4th axis gear...…..the idler diams also need to be big enough to not stress the belt due to a pointy back bending and if the belt slackens from stretch etc the adjustment also needs to allow the idler to get closer in.

I think that you only need to have more belt radial contact on the driver and driven pulleys if they are small like as in the EBAY 4th axis offerings at 1:6 reduction with a very small drive pulley and a large driven pulley...….that setup would benefit from a largish idler or two between the wouldn't want to attempt milling even aluminium with one of those......wood carving perhaps.

At the end of the day how much tightness is sufficient to just cater for indexing accuracy.....I don't think the belt will stretch in just turning the 4th spindle while indexing......the brake you favour will no doubt hold the spindle once in position against milling cutter forces...…...but when you mill on the move the brake won't be functional so the spindle can kick back.....probably not a problem on a finishing pass.

I considered the aspects of milling with a 4th axis for quite some time and decided on the resilient worm drive as it covers all aspects of both indexing and milling without having to add a braking force etc.