Food for thought: Actual incentives to tier up. Once you get to T3, you can do anything and everything a T4-6 company can do short of field a war fleet, which only matters if you're a mercenary group of some kind or another. Offer actual, tangible incentives that people would want to do 100-200-300+ posts for. Yeah, RP should be its own reward, but I'm just going to say the unpopular opinion: some folks want shiny things for the effort they go through. I'm not one of them, but there are folks that do that.
Also, don't just reward a major project. I could drop and have dropped hundreds of posts at least on expanding Tenloss across the galaxy. I have no desire to tier up, but if I woke up tomorrow with the urge to tier up to T4... All of that effort? Doesn't count. Not a bit. Drop X amount of posts to literally put your company on every planet, both on and off the map, in an entire sector? Never once considered as a project of any level for a Tier advancement.
TL;DR - There's no benefit outside of "nyah nyah, I'm Tier 4/5/6 and you aren't" for advancement in Tier short of fleet size which no one bothers with. Something tangible that folks can actually utilize/enjoy semi-frequently is key here. Also, broaden the spectrum on what constitutes a project. Just because someone dropped 400 posts in six months to take over a sector/X amount of rival companies/expand along an entire hyperlane doesn't mean its not equivalent in effort, scale, and impact as someone who dropped 400 posts on a brand new whizzbang to sell to the next schmuck for make-believe money.