OP, I brought this issue up in a thread a few months ago, when I was about the same stage you are now and was greeted with the same mix of responses. The fact of the matter is some people are considerate and will offer a seat but a significant proportion, probably majority, are selfish and inconsiderate, you have come across a good few on here.
I am now visibly heavily pregnant, over 8 months, and not much has changed, people will look directly at you and look away, and let's face it it's not because they have an invisible disability, they are just dicks. I don't care how anyone tries to rationalize their selfishness if you see someone who has something that means they are prioritised for the seat you are occupying, whether that's a baby badge, a baby, a walking stick, a limp or are elderly and you don't have the decency to give up your seat, you are a dick. And if you are in a priority seat you are an epic dick!
Afraid you will just have to ask and shame them into some semblance of human decency. I'm so fed up of asking at this point, I'm almost verging on rude. Before anyone goes on about 'hidden disability,' I doubt a mid 20s guy carrying a huge backpack, who clearly works in construction has one.
And yes pregnant women are entitled to the seats no matter how far along they are, otherwise they wouldn't issue a bloody badge until you are a certain amount of months along. Your pregnancy does not dictate what other women experience in their pregnancy.
Hope your journeys are a bit more comfortable, just build up the courage to ask, once you start you will get more used to it. Hopefully, you will mostly commute with people a lot more empathetic to pregnant women than some of those here on MUMSnet.
Take care!