Attic remodel with roof ventilation issues

Assuming modern 2x8s:

Purchase sheet foam insulation (you want "best" so polyiso gives the most R per inch as far as I know at present)

That's commonly available in 2 or 4 foot wide sheets 8 feet long, perhaps different or perhaps not so different in metric areas.

Trim to (measure to be sure, your roof may be less precise than would be convenient) 22-1/2" to account for "modern" 1-1/2" wide by 7-3/8" deep "2X8" on 24 inch centers. Either buy 6 inch thick sheets, or buy enough sheets to add up to 5 or 6 inches thick for this part, leaving a 1-3/8" to 2-3/8" gap for ventilation between the insulation sheet and the roof sheathing.

For best results, use another 1/2" to 3/4" full sheets of Polyiso over the face of the rafters before sheetrock to provide a thermal break.

This will cost considerably more than cellulose or fiberglass, but it has better insulation values and provides ventilation.

There are other (less expensive to install, less insulation value so more cost to own long-term) approaches, such as using ventilation baffles stapled to the roof sheathing (underside) in each bay for the ventilation, and then using "6-inch" rockwool or fiberglass - cellulose is also an option, but trickier to install - I would still suggest spending for an overall layer of foam covering the face of the rafters as suggested above. If your local retailer does not stock 24" wide batt insulation, discover the joys of online ordering, or consult a real building supplier rather than certain blue and orange retail box suppliers.