I have worn 509 helmets since like 2013. Bunch of different models. Just picked up a 509 Altitude 3K Carbon helmet. The thing fits my head awesome just like all the ones of years past. Has the Fid-0-lock which is an amazing little feature. Also, the new 3k carbon version is stupidly light. Wore it 2x now one some quick yard/road rides and cannot wait to get out and spend full days in it.
Vents awesome but if you need it a little warmer, just add the windproof liner.......If you need it warmer after that, you can add small foam pieces to the interior vent ports, then put the liner in, thus sealing it up really good.
I have the Suomy MX Jump. Just a bit over 2 lbs. Went from a helmet that weighted over 4 lbs several years ago. What a HUGE difference in how the ole neck feels at the end of the day. These are a little expensive but discounts can be found.