Sounds like an expensive project! I’ve attempted several floor leveling projects in our 110 year old clapboard house with its floors being angled this way and that due to joist sagging etc., and have had the most success combining a couple of approaches. First I would definitely remove the new floor (d’oh!). Then cut out the subfloor in the worst affected areas, i.e. the most saggy. Sister in new joists with lots of glue and screws but make the new joists as level as possible, that way you can avoid fiddling too much with shims, etc., although getting the new subfloor even with the old does take some finagling. There will almost certainly be areas where sistering is not possible due to other infrastructure in the way, so it’s up to you how involved you want to get with that (moving electric, plumbing, ducts, etc.). Sometimes you can sandwich plywood on either side of an existing joist to strengthen it and that takes up less space and works pretty well too. The main idea here is to add strength to the joists and also level them a little bit.

Once that’s done, and the new subfloor has been added, you can use leveling compound to level the floor out the rest of the way, and you won’t need to worry as much about the added weight from the leveling compound. I’ve only done this in areas where the floor was going to be tile or carpet although I believe this will also work with engineered wood flooring (but not regular hardwood).

Good luck! This is a big disruptive job that will take a while, just be careful and don’t fall through the floor.