I had a similar experience with DS2 - we were apart for about 24h as he had to be transferred to another hospital and they gave him what I felt were massive amounts of formula so my supply struggled to keep up for a while.
He is 16 months now and still very much breastfeeding. We stopped giving formula at about 4 months but could probably have stopped earlier. We just carried on because DH had come to enjoy giving the evening bottle feed. So don't worry, it absolutely can be done :)
What I did was:
Every 4 hours (day or night) or whenever he requested in between, put baby to breast. Allow him to take as much time as he wanted but look out for that switch from active sucking/swallowing/"schoop, schoop, schoop" sound into fluttery, asleep sucks. At this point swap sides. To be honest he was very sleepy, and I would usually have to change him or get up and walk around the room with him to get him to wake up enough to take the second side. When this changed to more fluttery sucking, that was the point at which I'd give the top up.
I was always careful to give the top up in the way of "paced feeding" - this is a method of bottle feeding which negates the way that a bottle uses the baby's instincts to keep them drinking. So you hold them almost sitting upright, as close as seems to be secure for them bearing in mind they can't support their own head. Bottle as close to horizontal as possible. Let them take breaks and allow the pace to be very slow. This way sometimes it would take him a really long time to finish a very small amount of formula/EBM, but I didn't mind because I felt this allowing him to go at his pace was important. BTW, it doesn't matter if you haven't been doing this beforehand - none of the SCBU nurses did it for DS2. He didn't seem to mind. We were in hospital, so I had to note down how much he took for the top up. They wanted me to aim for 30-50ml - I just gave him as much as he would take using the paced method. Sometimes it was 50ml, sometimes it was 10ml. I trusted that if he took less it was because he was full enough from my milk - easy for me to do, because he is my second and I'd breastfed before, but I would urge you to try and have this trust in the partnership of him and you, and avoid that temptation to just get him to have a little more milk. If he is hungry, he will definitely let you know so don't worry.
The reason you want to stop breastfeeding when you get to the fluttery sucks and move to the top up is that it's important he gets enough milk to keep his blood sugars up and if you allow him to spend too long at the breast he will be expending more energy than he is taking in. I did not appreciate this immediately, and I felt cross at being given a time scale (I was told 10 mins max each side - I believe this to be wrong and unhelpful, but might be useful if you're struggling to differentiate between different "types" of suck). But in fact, it's not hugely important to keep him on the breast as long as possible. Regular stimulation and removal of milk is important, not the length of time you keep him at the breast.
After this top up he was usually very sleepy so I could put him down. If you like at this point, you can express - ~10 minutes each side. You probably won't get very much if any milk from doing this, that's OK, it's not the point of the exercise. If you prefer, you can try expressing about an hour or two after a feed, but honestly, if you're getting him to the breast at least once every 4 hours - that's enough stimulation, prioritise your sleep and meals over expressing. You will also drive yourself crazy trying to keep up with the amount he needs. Easier to let yourself off that hook.
In 4 hours or if he woke up and cried earlier, repeat.
In addition to this - if I knew I had a 4-hour (ish) block where I didn't need to sleep, eat, shower or anything else I would make time to have some skin to skin, both of us under a blanket to keep him warm, I tried to do about an hour a day, I found the mornings were often best. At other times, between feeds I would also hold him as much as I felt like and let him have essentially free access to boob just for the extra stimulation/milk removal for me and extra milk for him. I would let him sleep on me and cluster feed, if he wanted to, and I didn't worry about the fluttery vs "active" sucking or give a top up for these feeds. But once every 4 hours at least I made sure I did a "proper" feed which was what I came to think of the whole rigmarole of changing him, feeding and watching the sucking pattern and doing the top up. I thought of the in-between boob access times as being "bonus" feeds and I didn't worry about noting them on the hospital sheet (we were in for 7 days). Sometimes he'd have a "proper" feed after 2 hours, sometimes 3, sometimes I had to wake him after 4.
After a few days of being at home, I was finding the topping up after each feed to be more cumbersome now I had to think more ahead than just ringing the bell for a nurse, and we knew roughly how much he would take during a day, so we decided to change the top up to a single bottle feed given in the evening which DH tended to do, and other than that one bottle feed, I just fed him as much as he wanted myself and I'd offer after 2-3 hours if he hadn't requested any. I let him stay as long as he liked after swapping for the second boob. I stopped setting alarms to wake up after 4 hours at night, as he was now alert enough to be waking me about that often anyway. I also co-slept by choice which again I was comfortable with after having done so with my first. If I felt like I was having a low supply day, I made sure to put him to the breast every hour on the hour (during the daytime) and things always caught up within 24-48 hours.
As I said, we ended up keeping the bedtime formula feed for a while as it provided quite a good start to a bedtime routine, but after a few months he seemed less keen and so we took that as a cue to stop. Since then he has been exclusively breastfed until the introduction of solids :) It's absolutely doable. My mature milk didn't come in until day 6 or 7, which I think was to do with the separation, and concerned me at the time but I needn't have worried. I also found the emotional hormone bit post-birth was a bit delayed for me, so watch out for that - I got really weepy and down during the second week in particular. But it passed, and we are OK. I'm sure you will be, as well 