Oracle Database Block Size
This db block size is causing our DB Buffer Pool to be constantly
  filled up (95%~100% full)


Your DB Buffer Pool should always be full unless you have recently restarted your server - Oracle will not purge blocks from the cache unless it has to in order to make room for other blocks. 

This in itself is not an indication that changing the block size will improve anything. You need to know more about the cache misses. 

If the OLTP side of your workload can query any part of your 5TB data, optimizing the buffer pool a little for it will probably not make much difference (but may unnecessarily slow down your big sequential scans that it was presumably optimized for)

On the other hand, if there is a small subset of the data hit regularly, and cache misses for the OLTP workloads can be substantially reduced by reducing the block size (which will depend on whether the reads are really scattered across blocks or not), then perhaps the benefits would be significant.

In the end you will have to test and find out :)