Ive been in the same situation when I first got my f5.. I only had some prime s35 lenses.. shoots where a zoom was just going to be vital I used my ENG zoom.. with the centre crop Abakus adaptor.. the s35mm ones were about $6K .. very heavy and ate light.. non starter for me anyway.. also dearly loved my very trusty Fujinon 4.5 HD zoom lens..
But my quickly I found it wasn't really cutting it shooting only centre crop and not getting the Shallow DoF "look" when I wanted it.. plus it was never going to pass muster for well paid shoots, turning up with an ENG zoom lens.. I found the 19-90 Cabrio not wide or long enough.. .. for a, at the time close to $40k lens... luckily the Canon CN7 came out as an answer to my prayers .. the draw back being it weights a ton !!..
Great for your f900.. but if you do actually go over to s35 sensor.. in my opinion .. for general freelance work.. and you want to stay with a servo zoom.. I think there is just no alternative but to get a s35m ENG style zoom..at least now there are more second hand ones around.. first few years ,buyers just didnt sell them at all..