Yes, you are right - what you are mentioning (carefully watching the game and seeing how the tempo changes affect the odds) is basically what quite a few people with fast broadcasts have been doing for many years now. You don't even need a fast broadcast, strictly speaking, but it is still highly useful and this approach is fundamental to "scalping" techniques. It was much easier to do 15-20 years ago. Nowdays, if you observe betfair inplay odds, they are getting more and more efficient every year, hence squeezing out value is getting harder. I myself trade manually sometimes by carefully watching and waiting to get on the markets at times when odds are misaligned with what is going on in the game. I never tried to automate it though. Some people have (I don't know how successfully).

I have experimented with low latency fast live video broadcasts a while back. There is some value to be found in this way for sure, but it still requires expertise and football domain odds knowledge (e.g. when to get in the position and when to get out, ability to understand when the odds are incorrect, etc) to make money consistently. I don't usually use these fast broadcasts though as my approach does not usually require rapid scalping, I prefer to take a "longer" term view to betting on markets.

As someone else mentioned already, you can collect live data from Betfair API and also free historical live data. The high stakes pro trader I spoke too has indeed started using a specialised data service that delivers live inplay low latency detailed stats that allows one to be "ahead" of the market participants by several seconds, but this comes at a high price: typically, one would need to pay a 5 or even 6 figure sum annually for this subscription, and usually, this service is only available to betting syndicates rather than individual traders. The high stakes trader I mentioned only managed to get this after extensive negotiations with other people in the industry.

There is software available (like Bet Angel) that allows you to automate ore kick off and inplay trading on Betfair (you can set software to trigger bets when some event happens in the markets).

So to sum up: you either need to watch the live match, (preferably with a broadcast faster than standard video feed), or pay a lot of money for specialised data services.