I totally get where you're coming from. Also don't forget, if the helicopter is going to be a milestone reward, there is a huge chance some sort of vehicle will be one as well. I think an ideal situation would be the core mechanics that influence the overall game being finished first before tackling one of these new game mode systems. Luckily we're seeing majority of that being done with the Summer Update, however there are still a few systems we need like achievements.
You're right about the update pacing, if there are going to be huge gaps in updates, you want to do it while the community is small. However keep in mind a lot of people are quick to ignore the tools for developmental insight and to write early access games off as "Dead" if they aren't seeing updates on Steam. Even the survey results show a lot of people rely on Steam to get their update news. So if SDNL can get all the art assets knocked out while Summer Update/Achievements are worked on, then be developed relatively quickly, that could be a good way to show users the game is expanding still on all fronts.
Also I'd rather see Arcade be tackled before a mechanically new game mode if the systems made for it could be used in the upcoming Condo IO tools. Would add more gameplay variety, improve plaza, help with the casino problem and potentially help condos. Also if the 2D arcade API is finished thats yet another way players can import their own content to the game (and one of the most exciting ways). While Arcade is probably a bigger task, it's been worked for a while now. I also think Arcade would draw in as many, if not more, players than Accelerate. Again, I'm slightly biased and these are opinions.