Remote working jobs are your best bet for gaining without being fired. The downside is that you won't get all the colleague interactions.
Corporate, Creative and Science jobs have the lowest activity levels of the other normal job types, so will let you work for much longer than, say, Fitness or Law Enforcement.
The two special job types, Dizzy's and Gentle Meadows, both let you work until immobility (similar to how it works for the NPC staff).
I'll take a look at working on the shop floor at a Dizzy's you own and make that possible to do on a mobility scooter!