Boiling it down to the three primary levers
- Ride for longer
- Ride harder
- Ride more often
But your case needs more nuance, most of which has been touched on already I think.
The two that come to my mind:
Are there particular routes that you could plan and ride that are better (with this objective in mind) than others?
Can you do some indoor workouts that deliver more load / hour in the time you available? For example, I use the TrainerDay HR+ feature to ride at my “Maffetone HR”. It delivers around 50 TSS / hour and is very sustainable for extended periods of time.