I'll confess up front that I haven't read all the replies yet, but while I had my two cents worth fresh in mind I wanted to jot it down, so I apologize in advance if someone already covered this in detail.
Technique--yes, absolutely. Get it down until it's practically muscle memory. But . . .
Prep: You mentioned you used electric clippers to trim down a bit prior to shaving; you also mentioned your beard is fairly coarse and perhaps as a result the Derby's tugged and the Astras didn't leave you with warm fuzzies. My hunch is your beard isn't quite prepped enough to properly hew to the blades edge. Good prep is perhaps more crucial than some realize. On average facial hair needs to be hydrated a full three minutes to be considered ready--as with everything else this may vary from person to person. But make sure your beard is well hydrated, perhaps consider a pre-shave oil? I've never used one but my understanding is they can help considerably with tougher growth.
Derby Extras worked surprisingly well for me . . . much better than I ever expected given the many poor reviews, but my facial hair is probably average--certainly not tough, but not fine either. Astras are my baseline against which I compare other blades--yeah, I know that's rife with its own issues, but anyway, I like Astras.
Lots of sage advice I've read above: try and resist the temptation to sample different hardware for at least a month; not an easy task, I grant you. Also consider mapping your beard grain. You may very well find out your results improve substantially if you do.
It is definitely far too early to throw in the towel. I had been shaving with carts for over thirty years, and it took me around a year to master the technique needed for a safety razor.
The razor and blades you have noted are all fine, and an experienced safety razor user should be able to get a good shave with any of them.
To learn the technique quickly, I strongly recommend the Gem razor system. This razor is like a safety razor with training wheels. It sets the angle for you, and it has extra thick blades which destroy even the heaviest stubble. Use it like any other safety razor, but put it large face flat on your own and you will instantly have the perfect angle.
You will quickly get the hang of it and then you will be able to transfer the newly mastered technique over to the DE format razors.
Since I have vacation I decided to treat myself with trying a new blade. Went for the Gillette 7 O’Clock Super Platinum (black) and I'm astonished.
I've only used it once, but it felt like a good combo with my Mühle R89. It was a bit sharper than the Astra but strangly it also felt really smooth. And it was slightly more effective in my eyes based on the results.
I don't think I'd hesitate to get more of these at some point for my Mühle.
The Muhle R89 is a moderate aggression razor. Thus, as long as your technique is good, The 7 O'Clock Black is a great choice for that razor. These blades are not cheap, but since they are both sharp and smooth, I think they are worth the price.
Now if you were using the aggressive R41, the Black might be too sharp and the less sharp Russian Green or Yellow might be a better choice. I have sensitive skin and prefer the 7 O'Clock Yellows in my R41.