Can't get 8 month old down to start sleep training without hysterical crying which leads to vomiting

Currently trying to sleep train our spirted 8 month old via ferber, and would love any advice on how to get him down without having him get hyterical and ultimately throwing up.

Current schedule is 2.75/3.25/3.5, with a wakeup of 6.30am and (attempted) bedtime of 7pm. Offered plenty of milk prior to bed, and is on three meals of solids during the day. Bedtime routine is close curtains together, read, calming song, sleeping sack, white noise on, brief cuddle/rocking and then down. He generally starts fighting it after reading, and quickly progresses to hysterical crying once put down....

The two times that I've tried sleeping training over the last couple of weeks I've been leaving him for 3 minutes, checking on him, leavig for 5, checking on him, but then generally don't get to the 10 minute interval as he hysterical and has ended up throwing up. My check-ins have done little to calm him.

Any suggestion as to how I can avoid him getting so hysterical?

Without it, he's currently sleeping for stretches of an hour in his cot, before ending up back in our arms (which is the case right now 😅) or falling asleep feeding and ending up co-sleeping (which is more often than no, fairly distrupted too).