I'm not going to go through the entire book point by point, but here are some salient points your summary misses:
One of the main points of the book is that when most people complain that life is short and they don't have much time they are mistaken. They are mistaken for various reasons, one of the main ones being that they waste their time.
Seneca would not agree with you that merely spending time "on whatever is meaningful in your life" would necessarily be a good use of your life. He goes on at length to decry the many things that even exceedingly rich people occupy their lives with that he thinks are a waste of time (such as vanity; chasing fame, wealth, or favor; being busy-bodies; concerning themselves with trivia, etc), and then talks about the things he thinks would be a good use of one's time -- the best according to him being (Stoic) philosophy.
He emphasizes that one should live for today, rather than for the future. "The greatest hinderance to living is expectancy, which depends upon the morrow and wastes today." "Everyone hurries his life on and suffers from a yearning for the future and a weariness of the present. But he who bestows all of his time on his own needs, who plans out every day as if it were his last, neither longs for nor fears the morrow."
So chasing wealth in order to at some future time have the leisure to enjoy it would be antithetical to Seneca, and is in fact one of the things he explicitly argues against:
"You will hear many men saying: “After my fiftieth year I shall retire into leisure, my sixtieth year shall release me from public duties.” And what guarantee, pray, have you that your life will last longer? Who will suffer your course to be just as you plan it? Are you not ashamed to reserve for yourself only the remnant of life, and to set apart for wisdom only that time which cannot be devoted to any business? How late it is to begin to live just when we must cease to live! What foolish forgetfulness of mortality to postpone wholesome plans to the fiftieth and sixtieth year, and to intend to begin life at a point to which few have attained!"
He's not encouraging the reader to chase after any get-rich-quick schemes either. The man upon whom fortune smiles today could be crushed under its foot tomorrow. Self-mastery and tranquility of mind is what he's after, as only then can one be indifferent to fortune's whims.
There's much more as well. This book is just so chock full of wisdom, I can not praise it highly enough. I really recommend you sit down one day, read through it slowly, and carefully think through each of the things he says.
> I just read it a few weeks back and was disappointed.
Too short, was it?