I have a loft that I want to insulate better, but also use as storage
space - so I don't want to lay deep insulation roll loosely on top.

It is currently fully boarded, with a bit of old fibreglass between the
joists. These are about 4-5 inches deep, holding lath and plaster ceilings
below. I have several LV lights in the ceilings below, which poke up
into the void I want to insulate.

The boards are currently up for other reasons, and here are the options
I have considered before putting them back down:

(1) Do nothing (but the fibreglass looks rather badly layed and isn't
filling the space well)

(2) Replace the old fibreglass with new insulation and put the boards
back. I guess Celotex blocks is the way to go? They could be about 4
inches deep. Can these rest on the lath and plaster ceilings or do
they need their own support? Or is the fibreglass just as good?
Would obviously cut around the LV lights.

(3) Make the joists deeper by adding 2x2s or 3x2s on top to get more
insulation, then reboard. This is probably too hard, and I'm hoping
4 inches of insulation is enough.

(4) Add a new layer of in insulation on top of the existing boards, and
lay new board on laths on top of this for my storage space.

A builder mentioned a new insulation product that was very thin but
effective, but not sure what he was referring to.

Any thoughts?