It is indeed hard to give good advise when we do not know more about your usage and the other factors that are influencing your decisions. My immediate suggestions would be:
1/ Crown RT3500 (Only low lift sit-down rider that I know of, but I think it is only marketed in Europe?)
2/ Maxtruck.se - ugly looking beast, but no mast!
3/ Jungheinrich EFG D30 - with a rotating operator cabin. Not sure if they still market it, but if you offer to buy 100 perhaps they will??
4/ Have a mast supplier engineer a custom mast for you - can't be too difficult to replace the standard masts?
5/ Add laser scanners as used on AGV. Suppliers like Sick, Keyence, Omron and others have multi-segment scanners that can be set up to slow the truck down as something enters the scanned field in front of the truck. It is a pretty idiot-proof solution, but in reality you will likely then also need to limit the top speed of your trucks. If you are to be able to safely bring the truck to a stop within the scanned field, that field becomes unpractically large if you go 15 mph...
6/ Replace the forklifts and its operators with a fleet of AGV

Hope this gives a few ideas to explore..?