you can try an mmo mouse
lots of buttons for you to setup as desired
i use the keyboard for wasd, shift (run) ctrl (crouch) and space for jump
the rest i can bind to the mouse, but i have loads spare, id not like it but you could add wasd on it if you wanted to
i use the corsair scimitar pro
it has an extra 12 buttons on the side (different profiles for different games)
there are different options
look for mmo/dota mice, they are the ones used to free up all the other keys
voice attack iv heard can be good too, assuming your not talking with others while you play
not sure if a pad would be an option for you, but the xbox controller is quite good for gaming
perhaps even a joystick if you can get to grips with it
im an old fart, i can only use keyboard and mouse, but i have a joystick i can see as usable
https://www.overclockers.co.uk/thrustmaster-t-16000m-fcs-usb-pc-joystick-pc-2960773-gc-073-th.html
it has enough to let you cover all fps needs button wise, but again you would have to be comfortable using one for aiming with, if your used to a mouse it might take some time
there are duel joystick options too, if thats easier
its worth a thought
have a look into them, i can confirm i used to know good fps players who use all of those options, one handed the joystick or the mmo mouse would be my recommendation