normally DIY co2 can use for around 1-2weeks depending amount of yeast you put as starter
Yes shaking the bottle can produce lots of bubble, then after that you will get even less bubbles than before you shaking it. Untable of co2 will result in lots of algae plus plants not growing well

Time to get a co2 tank !!!

This question you have to ask yourself
1. Do you love this hobby since you start planted tank?
2. Do you mind doing all the process DIY CO2 every week?
3. How is your budget?

1 set of co2 tank (4kg) + regulator is around rm250
Let's say u use DIY co2, each week 1 pack sugar + yeast = rm3, 1 year = rm150, 2 years rm300
For co2 tank, starter rm250, refill 2 times within 2years = rm50
When you quit, selling back at rm150 for whole set co2
Do the calculations