There are lots of extra large seat bags that will fit on any bike (even racing bikes) if you need to be self-sufficient for a big day out. Some of them are quite affordable, though they may not be as feather-light as high end bikepacking bags. For instance:
https://smile.amazon.com/Banjo-Broth...-no-redirect=1
If you really just want to carry a jacket (beyond your current seat bag's capacity), frankly, I would just get a jacket that packs nice and small. They are great to have anyway and you won't be weighed down by extra-large saddle bags, or handlebar bags, etc. Bonus points if the sleeves come off (convertible jackets) because the sleeves take up almost no space and their removal greatly increases the temperatures at which they can be worn (as the day warms up).
Happy riding!
Lots of good advice above, too, some of which surely echos my own post.