Whatever you have in TWS is the same you can get through the API. So if the data is live for free in TWS, it's also available in API; if the data is only delayed (no subscription) in TWS, it's not available in API.
We have an almost identical experience, although mine is in Python. First I wrote the basic data gathering and order placing calls, I thought that's it, I'm almost done. Little did I realize that was just about 25% of the job - timezone adjustments, dividend/split adjustments, tick size functions, currency conversions, making sure different strategies keep track of each other when they need to (while having different clientIds), halting functions when connection issues occurred etc took 75% of the time.
They do provide standard FIX (though they are a bit behind the most current version) but you pay quite a bit as retailer. Usd 1500 per month in minimum commission (obviously this is a moot point for anyone exceeding that that commission level) or so if I remember correctly. I was on FIX years ago but we completely abandoned it due to version inconsistencies between brokers and the time it took to adjust to broker specifc tags used or not used. Apis are way faster by nature and a good developer writes a new api interface as fast or faster than someone having to go through the entire FIX qualification process. But that is just my own opinion. By the way for any quirks that are not mentioned in the official IB API documentation the best resource by far is still the very active TWS API user group https://groups.io/g/twsapi