well the first thing to do is tell us which vendor you’re using. Yes, each vendor provides several counters related to the number of pdch_used and tch_used, and perhaps even the number of peak tch/pdch used in the hour.
How mature is your network ? Do you have U2100 and U900 deployed continuously along your 2G sites ?
If that’s the case, you could probably decide to reserve only 2 static PDCH (in your capa analysis) on top of the required nb of TCH. That provides enough capacity for the few users that are still connected to 2G.
On the note of power reduction, there are features that switch off TRX when they are not used, and decrease by few dB’s the TRX0 whenever possible. So you may want to consider that as well.