You can generate a lot of speed by just being smooth and steady. Not too hard on the uphills and into the wind, but not too easy downhill or w tail wind. With the cv19 thing, lots of solo riding and I’ve been surprised how high my average speed has been on long endurance rides when I focus on holding consistent power. Often faster than group ride speeds on same route with similar normalized power. Group rides can be very surgy and that’s not the way to go fast. Wind and hills also have a big impact. Uphill and headwinds hurt your average speed more then the corresponding downhill and tailwind help. A couple weeks ago, I did a solo century and averaged 21.2 mph on only 235 np on a day w really low winds. Same route the prior week w moderate winds and only averaged 20.6 on the same power. I’m always much faster on light wind days.
I think the biggest thing for me is probably going to hard on climbs and not leaving enough to power over the top. I end up riding zone4/5 on climbs then doing zone 2 to recover for a few minute then back to tempo. I’m thinking I need to maybe adjust my riding style.
What distance are you trying to do at 30km/h? I was similar to you (75kg, FTP around 320W) last season and was able to do 100km with around 1000m of climbing at 30km/h by holding 230W.
Tempo and SS are pretty sustainable for long rides. I did an imperial century last summer at an IF of 0.87.
Your watts to speed ratio seems a bit off though. Are those heavier riders on the same roads as you? Riding solo or in groups? If it’s an apples to apples comparison i.e. they’re riding similar routes, solo, but are faster than you with more kg and fewer watts, then something isn’t right. Either you have some relatively easy speed wins to be gained - changing your position, riding on the drops more, snugger clothing, better tires, etc. Or your PM (or theirs) is off.
Route profile, temperature, wind, road surface all make a huge difference. Difference between a loop on a flat smooth road with minimal wind vs a rolling route with some wind and a rougher road surface could easily be 3-4mph at the same power.
As a 185 cm tall rider weighing 90 kg two things stood out to me the most when looking at avg speeds lately.
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watts — increasing your threshold will allow you to just plow through the wind. Recently did a nice easy week of steady state riding in z2 after 20 weeks of doing TR plans. Prior to starting the plans my avg was around 30-31 km/h when doing the same loop rides, now I am averaging 34-35.
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position — this is very likely a big part of the recent increase in speed. I have been working on trying to sustain the bent elbows position which is said to be the fastest of all positions. Initially I could only hold if for 20-30 seconds before going back into the drops and giving my arms a rest, now I can hold the position for 2-3 minutes with only needing small rests in between on the drops.
Edit: if not in the bent elbows position, then I am always in the drops. This I worked on last season, so now even a 2-3h ride fully in the drops is no biggy. If you can get comfortable in the drops then you will see your avg speed improve quite significantly.
Edit 2: Routes I ride are mainly flat-sh roads with good tarmac with only a couple of power hills here and there.
So for me power and position is king 
FWIW, if you’re not doing much climbing I’d focus on position. I’m the same weight as you are and 188cm. Position on the bike makes a huge difference for me, especially on the flats.
I’d keep working on your fitness and start making small improvements to your position, rather than worry about an average speed. I went on a VERY long ride this weekend with LOTS of climbing and averaged 31kph at 220w avg on a windy day. But, the only way I held the speed was by being able to push tempo up the climbs and rollers hour after hour without blowing up because my threshold is quite a bit higher.
The numbers don’t always tell the whole story.
I’ll bare that in mind on my next ride. I’m currently on a build plan for another week but going onto the gran fondo plan after the recovery week so some more base work is in the line up. The climbs near me are only short maximum of 2.5 miles at like 8/10% but I can avoid them, but the terrain in My area is very rolling so I’m hoping If I can keep a higher average speed in this area then when flat I’ll be able to power through easier. This is the theory.