Welcome flipp.
I can't say that you will get fast and easy results in your quest. Improving in freediving is:
- learning to completely relax before a dive (wouldn't work for you)
- learning to breath fully but normally before a dive (dito)
- using some stretching/breathing exercises to increase your lung volume (would do little for you)
- spending a lot of time in the water (you already do that)
- learning to enjoy/controll the U/W sensation (may work)
IMHO tables don't help too much, but some people swear by them.
I would bet though that a freediving course would help, so much knowledge packed in to a short time!
Even to this day, after 18 months quite regular practice, if i dive unprepared I would say that I would do as well or even worse than you
