Dip assist machine. The more weight you add the easier is to dips. Eventually should be able to dips normally

I'm not sure what exercise would be a suitable substitute since the goal is to isolate the pec and you've already got flye's in the mix, but a good way to build up to being able to do working sets is by isolating the adduction of the exercise .. so the 'coming down' on the dip. i.e. use your legs to stand up into the fully extended portion of the dip and resist on the way down, use your legs to stand back up and do this again. Do them like this until you can't control the dip anymore (failure)

Use a spoter or a dip machine

most gyms have an assisted dip/pull up machine, try using that