The departure tax can be paid in Euros/USD. Not sure if you can get change in those currencies though. I find keeping a 10,000 CFA bill on hand when leaving to be the easiest.

As for lounge in DLA - do not expect much. AFAIK, there is only 1 shared business class lounge at DLA - shared by all airlines. It is not very nice but better than nothing!

I go to Cameroon about 5 times/year now for business, usually avoiding DLA. Yaounde (NSI) is a better airport experience and city, IMO.