I really think you should use the footage of the Cad Bane fan film to fill his absence in the series. Not too much of it, since the visual of Bane between BOBF and this fan film don’t match, but certain scenes. In the beginning of the fan film we have T0D0 talking to a jawa being held captive, and then the first glimpse of Bane is just his arm. This could be totally usable, since in one scene of BOBF was showed the jawas being afraid of Bane. Just insert some dialogue of Bane in this scene, to establish his involvement in the story of BOBF, and that’s it (maybe a few lines mentioning the pykes and Boba). In 12min48sec, there is a take of Bane’s eyes that is totally usable too. These kinds of takes that just focus on his arm or eyes could help to introduce the character, while still maintaining the mystery around his figure (which will show entirely just in his fight against Cobb Vainth).
Link to the fan film: