I switched to formula at 5.5 months, after a miserable time breastfeeding - DS had a tongue tie and fed every 2 hours for 45-60 mins at a time, and despite us having the tongue tie separated he continued to have very poor weight gain. I was immeasurably happier after switching, and his weight gain improved massively, and he has just thrived - he's now 20 months. I felt horribly guilty stopping, but on logical reflection and more sleep, I have no idea why. You've done brilliantly to get so far - if you want to stop it's your choice, and noone else's.
What I did was replace two feeds a day with formula (initially the late evening feed, so DH could do it and give me a break some nights), and a mid morning feed. I fed on demand, just as I had when BF. By that age, I didn't necessarily offer milk immediately when he woke, as he wasn't always hungry - I waited for him to show feeding cues (hand chewing, grizzling etc). At 5 months, he would easily take 8oz of formula at a feed. My plan had been to slowly replace more feeds, to avoid engorgement, but after around 2 weeks he started refusing the breast, and that was that. I was very sore, and had to express for comfort, but the pain went after about a week and my milk was gone after about 4 weeks.
His sleep did extend after starting formula, but I think that was because the tongue tie stopped him from ever really getting enough milk when breastfeeding. He used to fall asleep on the breast after feeding for almost an hour, then wake up an hour later, hungry again. With formula, I think he was able to satisfy his hunger - it certainly made a big difference to his weight gain, he had been borderline failure-to-thrive beforehand. For a baby who is getting enough milk from the breast, I can't see it making as much difference.
Good luck, and I hope whatever you decide you feel better for it - those first few months are so tough.