TTC but boyfriend keeps making a weird “joke” – am I overreacting?

Well he's right, isn't he. Have you thought about this carefully enough? You are absolutely tied to him for life once you have a child.. in one way or another.

Is there a reason you are not married? Does he not want to commit? In which case this comment makes more sense.

is it maybe a reverse? like maybe what he’s actually saying is the opposite of what he’s thinking, i.e. ‘once you’re pregnant i’m trapped with you forever’ i’d personally take it as him having nerves more than anything. that or an insecurity thing, e.g. are you sure you want a baby with me

It's not unknown for controlling men to maintain a mask until they feel their partner is trapped into staying in the relationship. Marriage or childbirth are the usual traps.

But it's probably unknown for them to give such a heads-up to their victim!

Talk to him. Tell him this 'joke' really isn't funny and it's giving you the ick (which he really should take seriously, but might not) and you need to know what the fuck is he doing?

They sound like his thoughts about himself. Listen to him say it as "I'll be trapped with you forever/you'll be trapped with me forever, are we sure this is what we want?" It's a huge commitment for the rest of your lives. Maybe try having a serious discussion about it when he next brings it up. We had a lot of conversations like this before having children. Are we sure we really want this immense tie between us forever. Imagine the impact. What it will be like in 2, 5, 20 years time. I think it's a difficult conversation to have without sounding like you're doubting your relationship together but it's important to talk honestly.

I could cope with the " no going back" comment. But I would be worried about the " trapped" comment. I would wonder if he is getting a feeling of being trapped himself.
I wouldn't ignore these comments. I would be having a discussion about why he is saying these things because generally speaking things presented as jokes generally are masking real thoughts and feelings.

Maybe he's just stressing about the magnitude of the situation. It's the biggest commitment you can make as you will effectively be tied together for life through a shared child, even if the relationship collapses.

Yes, he's projecting a feeling he is having about commitment. Which is perfectly acceptable and normal, this is a big life transition. It's easier for him if the feeling is projected into you, but better if he could resolve it in himself.

I'd be having a serious conversation about whether he's ready for parenthood. You will get people saying don't TTC unmarried, but my question would be whether you've thoroughly talked through exactly how this baby will be cared for once it's born, assuming you are not planning to become a SAHP, which would be lunatic? Who handles drop offs and pick ups to childminder/nursery? How are sick days going to be dealt with? How can you both make your jobs flexible enough to be able to care for a child? If you don't know the answer to all this, don't TTC.

Have you told him it's making you feel weird and vulnerable?

I don't think it's a red flag. My partner and I tease each other about being stuck with one another all the time since we bought a house together. We say it because we feel we know the other won't mind being stuck with us, which is why it's a joke.

But if it's making you feel dodgy, you need to tell him.

Maybe he's feeling nervous about the commitment?