Im Def on the "slim needs something" train.
A few things that I could see working for slim:
1 extra bullet in his magazine. With perfect aiming, this would essentially give slim 3 potential heal bursts per magazine with standard capacity.
Reducing the natural cooldown of his heal burst
Reducing the number of pellets needed to recharge his heal burst. Maybe enough so that 2 fully landed shots would completely replenish his heal burst.
(Obviously im not saying do all of these, and im not saying that these should be done without OTHER balancing considerations. Dramatically increasing the amount of heal bursts he can kick out over time without touching the amount they heal for would be bad)
Smaller things might be a bit more accuracy on hit shotgun (either flat out decrease in the spread, or a reduction in the time it takes for his next shot to come back to full-accuracy), and maybe even a damage boost. If im not mistaken rogue val basically easily out damages slim most of the time, while also having amazing self sustain, and arguably better team-healing, and she highlights the monster, and..