Plan your boards.
Make templates of the edges, when they are not just flat, so that you can make sure they'll fit together cleanly.
Cut foam with a very sharp bread knife or a retractable "snap off" knife.
Hot wire cutters and Creme brulee burners are ok but work better on expanded polystyrene rather than the extruded foam I think you have? If you're doing a lot of these get a certified chemical protection half mask like this: https://www.amazon.co.uk/gp/product/B00 ... UTF8&psc=1
Key point to remember is white "smoke" is predominantly water vapour and mostly harmless but any hint of black smoke and you're burning the foam and producing toxic chemicals.
Don't get into burning or sanding your contours down to smooth gradients. It's a thankless task and makes too much mess. use modelling compound/Sculptamould which are largely a mix of plaster and paper.
Do anything you can to avoid painting too much of the board! Mixes of fine aggregates, coloured tile grout and pigments can, when well done totally avoid the need for painting. Tile grout is also an adhesive so it will make your boards rock hard and wargames friendly.
Get a static grass applicator to grass your boards. Use PVA for the first layer then spray varnish for additional layers. Start with a 2mm static then rough it up and add patches of longer grass. but for wargaming don't go more than 6mm. Add thin patches of your aggregate/grout mix over the grass to break it up.
Static grass also works pretty well for disguising the joins between boards if you make sure the colours are always similar towards the edges.
If you prefer go for foam flocks, as they can be graded to give different ground effects and are much more hard wearing and colourfast than sawdust types.
Seal everything at every stage before moving onto the next.
Over a decade after producing about 40 x 2 ft square boards i'm just starting to refurbish some of them for my new 15mm projects so i'm also heading down the same avenues.
Happy to chat about this and bounce ideas around at any time
