I also carry a small saddle bag with the “every ride” essentials. On the longer events I do, I need to bring a few extra things.
One option I have is my hydration pack. This has a 2L bladder which more than doubles the two water bottles of water/mix I typically carry on the bike. Additionally, my hydration pack has an extra cargo pocket that can hold a few extra snacks and tools. Some people don’t like a backpack while riding, but it does not really bother me at all.
if that’s not enough, I might instead (or in addition) put on my half frame bag. I like that better than the hydration pack if I’m not needing to carry a lot of fluids. There are a lot of specialized frame bags if you look around...you can likely find one that’ll mount in whatever unused space you have in your main triangle...and if you cant find one pre-made, there are a lot of custom makers than can stitch something up for you.