Assuming that your heart rate is mostly constant during your runs then according to the setup zones you're running in zone 2. Zone 2 is not a zone to give you a high relative effort. If you feel that your effort is much higher than the usual zone 2 (your breathing in the "real" zone 2 would be mostly relaxed with no problems speaking or singing) then you should adjust the zones so that you come into zone 3 with these efforts.

If you should pushing harder is something that only you can answer based on how you feel before, during and after the runs. A fitness level of 137 is very high, but that only means you have exercised very often and long in recent times. The number alone can only give subtle hints if you're overreaching or slacking when you compare it to previous numbers.

I guess the real conundrum for me here is that the Strava "fitness" scale then doesn't seem to fit with the standard wisdom of the day. If I were to push harder for my 30km long run I would be getting up into Zone 3, which so many people say now is a no-no, but I would be rewarded on the Strava "Fitness" scale.

If the vast majority of the training world is saying "run slow to get faster", "build fitness by running slowly", etc, etc. Then why isn't Strava recognizing these runs as improving my "fitness".

Conventional wisdom right now is saying that the long runs are the most important ones, and to make sure they aren't too fast (that you'd get into zone three). But then Strava is saying...meh, you're 30k run was basically not a workout and didn't improve your fitness.

Maybe a Strava problem?

That's only the usual problem that everybody thinks at first, that Strava's fitness means fitness. This forum is full of that but they won't change the term and don't even comment on that misleading. Strava's fitness is just a training load and therefore you will get higher numbers for being in higher zones.