Most of the time she falls asleep on one breast and is wide awake on the second. Then it will take 60 minutes of getting her sleepy, putting her down, waking and resettling over and over. She goes down, sleeps for 10 minutes then wakes and cries. This will go on until the next feed or close to. It's always 1.5 hours after I start the feed that she finally starts looking sleepy. And feeds are only taking 30 minutes now. So that's 60 minutes of settling.
Are you waking her up to give her the second side? Have you tried just letting her sleep? If she falls asleep after the first side then maybe she is done. Waking her up and feeding her more might just be upsetting her tummy? Maybe just put her into bed when she falls asleep and then give her a top up feed when she wakes. There's no need to stick to any schedule with feeding, just your baby's schedule.

OR, you also said if she's not sleepy after a feed, then it's usually 1.5hrs later she is looking sleepy. Why not just lets her stay awake until then? There's no use trying to put a baby to sleep that isn't sleepy. Wait for her sleepy cues and start then. I always tried to put them to sleep with a feed because that was easiest. So I would feed when she woke, then if she wasn't sleepy after the feed just let her stay awake (cuddle her, talk to her, give her some tummy time, whatever you want), then I would feed again when she looked sleepy and let her fall asleep on the breast. I would then try and put her down when she was asleep. If she woke after 10 mins I would resettle by rocking or a sling, and if that didn't work just feed again. When she was asleep I wouldn't bother trying to put her down again, just hold her and watch TV or something and rest. Mine always liked to sleep on me and would sleep much longer that way.

You can feed her as much as you want. It's a really natural way for a baby to fall asleep (not to mean the easiest for us!) so I would forget about any kind of schedule.

There is a reason she is sleepy during a feed and then again a bit later, go with it:

the baby produces CCK (cholecystokinin), both from suckling and from the fatty hindmilk reaching the baby’s stomach at the end of a feed. The Australian Breastfeeding Association explains:
There are actually two CCK peaks, one at the end of a feed, and the other higher peak between 30 and 60 minutes after the feed. The baby sucks, gets sleepy, dozes off for a while then wakes again for a top-up feed. That higher-fat feed causes the second peak and the baby goes into deeper sleep. Top-up feeds are also great for the mother’s milk supply.