Re type of imaging: I think that a jump from widefield (400mm) to 900mm FL might challenge me to move on from the more commonly imaged targets - I think that would be a positive move for me. But, FSQ-type widefield images are the ones that appeal to me the most.
Their WOW factor is an addictive motivation for the pursuit of a sweet refractor.
As dilemmas go, it's a nice dilemma to have!
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FSQ images are among the most popular of all images for sure.
Refractor images to me are the most pure of images as no central obstruction, no diffractions spikes are those tight tiny stars make for very appealling images.
TMB92 may capture similar. Are there example images on the net you are aware of using that scope?
The TMB though isn't 900mm focal length. But you may be able to use a Tak 1.6 extender to make it about 825mm focal length and nearly F9.
I have used a Tak 1.6 extender on my TEC180 and it worked very well.
Greg.