Insulating window reveals

Thermal imaging shows that some of my windows suffer from significant thermal bridging. One solution would be to insulate the window reveals.

I don’t mind hiding some of the frame but I need to keep usable access to the glazing beads in case I ever need to replace the sealed units. To increase insulation space slightly and reduce thermal bridging further I am thinking of removing existing plaster and insulating direct to the brick.

Thin insulation which nevertheless has a low U-value is the order of the day. As far as I know that is aerogel territory. I have found a few companies that supply aerogel insulated window reveal board (I haven’t dared ask the prices yet).

Can anyone tell me whether all aerogels are equal, and if not explain the differences?

Does anyone have good experience with aerogel?

Any other advice on insulating window reveals gratefully accepted.

I used wood fibre that was then plaster skimmed. The boards are a standard thickness but may vary with manufacturer. I would suggest horses for courses- not all windows will need the thinnest material. I used Udi from Back to earth.