How to fix this floor

[JP_]

The screed in the new (3 years old) extension is crumbling, and I am almost ready to sort the floor out of the old kitchen that I've been renovating. Using the 18mm caberboards on existing joists.

Can I just put down a stronger screed / level mix? Builders did try to improve the work with some level compound mix but that is broken now. Will be cutting the vinyl back to the door stops. Will have laminate boards on new floor.

I am thinking the options are:

1. remove some of existing and replace with better - need to put in a barrier to hold it better too. Builders used a bit of 25mm insulation board on its side. Not sure how effective that is...
2. removing existing to door stop, putting in something to help hold it together on the edge, then board up to it
3. remove 2cm top layer to rest floor board on top, fill around etc.
4. Just board as close as possible, then when floor screwed down fill in with some sort of filler.

Or something else?

 

[JP_]

well, this was my attempt. Removed move crumbing concrete, put in a bit of timber to floor level, foamed around it so there's a bottom, used U-can concrete floor patch fill.
Hopefully it is strong enough and level enough.

Screwdriver not relevant, just there from removing the door - the big lump of concrete came away in my fingers.
Removed all loose (the hole on the right got much bigger) then soaked it in water, before filling. Also trimmed off as much foam as possible.

Obviously a bit messy, first time doing this. Insulation below future laminate board should hopefully disguise any unlevel bits ...