Heinrich,
The inability to receive DHCP could be an RF issue. We need to ensure that the RF environment is very good, before we attempt to troubleshoot anything else.
You should have your access point power at min 12 and max 18 on the 5ghz side. Why? That is so that a user will not hang on to an access point that is far out of the access point radius, because the signal is still good. having access points at the max power can cause roaming issues, because clients normally do not attempt to roam until the signal gets "bad enough". In addition, since your client has an open office space area, RF can travel hundreds of feet, dragging tons of clients into a collision domain, as a result.
You should not use DFS channels, UNLESS a great majority of your clients support it, because not all clients can see DFS channels, so they will probably connect to a sub-optimal access point, and not the one right above it, as a result. That would make that client connect to an access point further away and that will drag down the performance of all the users who are connected to that access point, because the client will take longer to transmit further away.
You should use 20mhz channels with any density so that you have 9 non-overlapping (non-dfs) channels instead of 4 with 40mhz (non-dfs). This is because all clients cannot use 40 mhz channels and that means when those clients connect, 20mhz of spectrum will go unused...and other clients cannot use the other 20mhz.
When you have high density, you might want to try increasing the basic and tx rates on that SSID so that the management traffic takes less airtime and reduces your utilization. by default the lowest management and data rate on the 5ghz is 6mb. You could try changing it to something like 18 for each, and your utilization will drop.
You also do not have to set static channels. Make enough channels available and ARM should do its thing.
Since any ip connectivity rides on RF connectivity, you will be able to see if there is an IP issue when you make the RF changes. It is hard to troubleshoot an IP issue, when there are still RF issues. There is only so much that Clientmatch can do.