Since you already own a Brompton I would try modding that one first; or consider a Rodriguez 6 pack. I have a Brompton M2LX with 58t chainring and 12t cog. I can can get up to 40kmh before my gearing starts to run out. I think 12t is the smallest rear and 60t is the largest front with the ability to fold. You could go bigger in front but then the fork/swingarm hook might be blocked by the chainring so you would have to use a strap or something to hold the bike together. Not sure if it will feel stable, but I would get a cheap flat mtb bar and clip on aerobars for less than $50 off Aliexpress so you can keep the stock brake levers. Road bars might be nice but doubtful it will fold small and requires new brake levers. If you spent a whole lot of money you could get your bike under 8kg.