You say you eat lots of veggies so you should be getting enough fibre, do you calculate & record the fibre? Like you say berries are a good source, mix with full fat Greek yogurt sprinkle some chia seeds & flax seeds & a few nuts like almonds & walnuts should give you a good amount of fibre on top of the veggies & keep you full for hours
You can sprinkle the seeds on salads & veggies too if you don’t want to eat yogurt
The fat from the yogurt & nuts should slow the rise from the few carbs in the berries.
I’m keto on high fibre for another condition too, I find I get more than enough of fibre I need from the above & my veggies, cauliflower, broccoli, asparagus, celeriac, cabbages, leeks, Brussels, mushrooms, avocado is good too