Eight Sub Band Filter Design
This sounds like the classical job of a polyphase filterbank, and a cascade of polyphase synthesizers. Yes, do this in complex baseband.

First, you use your filterbank to divide the 25 MHz in 25 kHz wide channels. Then, you use polyphase synthesizers to combine subsets these channels back to the channel widths you need, and then you use another one to combine these back to 25 MHz of bandwidth.

Another option would be to simply have a prepared table of filter coefficients for your potential subband widths, and just shift these in frequency (which is rather trivial) accordingly to get out your subbands of choice, then combine these back together with a bog-normal adder.

I'm assuming none of this requires that group delay in all these subbands is identical, for example. Otherwise, the design approach would have to amount for different filter delays.