Hi - I've been there and know how difficult it is, so well done for getting this far. I exclusively expressed for my son for 4 months from August, until he finally decided to feed direct from me! Typical awkward man!
Anyway, like you I used a medela double pump and expressed at least 4 times during the day and also whenever he got up at night for a feed (my husband gave him his night bottles whilst I expressed for at least half an hour) - expressing during the early hours of the morning is actually very good for milk supply as this is apparently when a lot of the required hormone is produced.
My son started on 30ml, then 50ml, then 80ml, 120ml and up to 150ml.
After the first couple of weeks, I found that my supply was such that I always had at least 2 or 3 feeds in the fridge and also managed to build up a frozen supply (enough to last for 2 whole days if necessary).
I did keep on trying to get him to latch every day. Sometimes he would feed from me, but it would take about 45mins and he would then be hungry again only an hour later (as opposed to 3hours between bottle feeds). HV and midwifes were no help and to be honest I probably didn't put as much effort into trying to get him to latch on as I should have done.
However, as he got to about 3months old it got much harder. Partly because we started to get out and about more so I felt I didn't have enough time at home to express and partly as he went through a growth spurt and wanted more milk. I was really starting to panic about it and concerned that my supply wasn't up to it. I started to take fenugreek supplements
And then one day I was out with only one bottle with me and stayed out longer than expected. He needed a feed and I had no choice but to try to feed him direct, which he did. This was our most successful "direct" feed as it got him through the next 2 hours!
I spoke with a breastfeeding counsellor who gave me a couple of tips and who gave me the confidence that I could do this. I just really concentrated on getting him to feed direct, spending a couple of days at home, lots of skin to skin, and something just clicked between the 2 of us.
I've only expressed twice since Christmas, when I was going out, but DS has now decided he won't take milk from a bottle anymore, so that was a complete disaster!!
Anyway, sorry for the essay - I didn't intend this to be such a long post. Expressing is possible on a long term basis, and your supply should adapt to keep up. But I would say to keep on trying to get him to latch and feed direct.
Good luck