Analyzing your figures, assuming your wheel circumference is 2110mm ...
At 25kph, riding the 34X14 gears, your cadence is about 81.
If you switch to the 50X21, your cadence is about 83.
If you then push your cadence up, at a cadence of 100 on the 50X21 you are doing 30 kph.
You can then drop to the 50X19 for a cadence of 90.
So this all looks pretty normal.
Since you are not complaining about hills, I say your gears are about right for you. You are complaining of feeling "in between" on the flat, so I have one and a half suggestions:
Practice riding with a higher cadence. It always feels weird at first, but you'll get used to it. If you start "bobbing", then concentrate on pedaling smoothly (ankling). Think of riding at a higher cadence like running a car at higher RPM. The higher the RPM the higher the power. And think how many steps per minute you would take if you were running. Not everybody would agree with me, but I say that 90 should be your slowest cadence (when you should change to a lower gear), with 100 as your average. For practice, try this game. Ride with a friend, starting at, say 20 kph on the 34X17 gear. Now in the same gear find out how fast you can go. You will be surprised. Then try the next higher gear, and so on.
You could test some 53/39 front rings. But you would lose more at the bottom end than you would gain at the top end: the gearing range is lower, with more overlap. To make it work, you still need to teach your legs to ride at a higher cadence :-)
Does it matter if I got your wheel size wrong? Not much. Does it take long to learn to ride at 90+ all the time? It can take a month or two. You have to train your legs to do it, and part of that is your circulatory system has to change. Both need time. What is the benefit? There are three benefits: you will overcome that "in between" feeling, plus greater efficiency and power. Enjoy!
PS You'll notice I didn't mention cross-chaining. That's because I don't think it's an issue.
PPS After posting, I realized I left out an option: to change the cassette to one with smaller gaps in the cog sizes, something like 12-25 or a custom built job. That won't work for you if you need that 27 cog for the hills.