:( :( hope you have a better day today, louey.
Your baby had a difficult start - born a little early, a stay in SCBU (which would mean fewer cuddles and contact with you than he would otherwise have, and contact with a range of other carers), tube fed (missing out on 'direct feeding' and the sucking comfort babies get with this) and difficulties actually getting milk out when he needed, hence the tongue tie.
So it's not a bit surprising he is gonna take a while to get to a calm, gentle, relaxed way of feeding (same goes for you - you shared these difficult experiences together).
It's worth thinking about going with the flow for the time being, as far as you can. Your baby is now term, and is still a very new baby, and you have never really had the chance to get close to each other without some problem getting in the way. A few days of snuggling up close to each other in bed if you can, or on the sofa at other times, and co-sleeping in the bed at night....is that possible? You'll need help and support with your other DCs for this to happen, obviously, and to feel confident things are going to tick over without you being able to be hands on. Partner, mother, friend....anyone able to do this for you? Can they take your DCs to their apppointments while you spend time with the baby?
Your baby may well just want to hang out at the breast without doing much sucking - and scream when taken away. That's ok. It's normal. He's trying to check in with you (he doesn't work this out, it's not a conscious thing, but it is a natural new baby thing which babies are 'hard wired' to do). Yes, keep up with the expressing and the bottles to ensure he is well nourished and avoid trying to 'get' him to suck....he will get there in time, and faster if you keep your times together unstressed and unpressured. If you can just let him, and yourself, relax and 'tune in' together without the anxiety of 'he must feed NOW' things will be easier for you and him....and breastfeeding will take care of itself.
A call to any of the bf helplines should help you explore these options.