Thanks very much to anyone who contributed to this discussion! I will here try to provide some additionial information/thoughts that maybe prolong the discussion. And I will also try to draw some conclusions of what has been said above, and point out how I think I will continue my training.
I started climbing/going to the mountains as early as 1990, but started more serious sport climbing only in 1994. Already in 1996 I could do my first 7a. For the following 10 years until 2006 I plateaued on that level. Went to the crags or the wall and climbed at my (perceived) max, 7a. Thus, didn't do any strength training, never tried hard redpoints, but just did endurance and collected mileage. Only from the end of 2006 on I switched to indoor bouldering rather than climbing, and also tried to redpoint harder routes outdoor. It rose my level up to 8a (first one in 2010). Since reaching that, my max ability was +/- constant.
From this background I draw the conclusion that strength training is what I should aim for, but not endurance. Being a rather tall guy (190cm), I am, despite being skinny and low body fat, rather heavy, too (78kg). I relatively suck on very steep indoor endurance routes with no good holds or shakeouts in between, that's not my cup of tea. On the other hand, I still think that endurance is my strong point: when climbing outdoor, I rarely find that it is the lack of endurance that stops me. Not even at Kalymnos, where the climbing is steep (but offers resting positions), and not at all hereabout (I live in Switzerland), where even the limestone routes are generally much more techy/sequency, but not so steep and sustained. Furthermore, I'm not interested at all in my indoor performance, only outdoor counts.
I found the discussion about whether I should ditch the endurance work or not very interesting. Maybe I should add that the training questions I asked are with focus on right now (i.e. winter), where I only rarely climb outdoors. In summer, I climb outdoor for ~2x per week. Endurance naturally becomes a more important and more worked asset then, but I need to be able to profit from my strength that I built in winter, as I will reduce the bouldering to ~1 session per week.
Science tells us that following a well structured training plan with periodization would yield the best results. That may apply to my case as well. But partly due to other commitments (family, job), and also because of a lack of motivation (I rather climb then train, and doing outdoor stuff is much more important to me than training), I would never want to follow a strict, scientific training plan that has first priority over all outdoor adventures that could be had. As explained in the opening post, the goal of my question was to think about what I'm currently doing, adapt it if necessary, or complement it with other exercises as far as possible.
Conclusions:
1) I predominantly need (enough) bouldering sessions where I purely focus on strength. This means taking proper rests in between attempts, so that the next one is better than the one before. If performance declines, then stop.
2) There can be bouldering (or sometimes also indoor route) sessions that focus more on the endurance aspect. But also there, I need to climb longer problems (or sets of boulders) cleanly and with focus, then rest properly, and start over. If performance clearly declines, then stop. Also, avoid stuping pumping by doing many easy problems without proper rests or concentration just to feel knackered after training.
3) Do some complementary training. I will try some fingerboarding, but most importantly, core strength exercises. I did some research on that topic (will do more), and what I found here (
http://climbstrong.wordpress.com/2012/02/12/core-training-for-climbers/ ) seems like a good set of exercises. At least I feel that I suck at all these, and that they could bring improvement in my climbing. Additionally, I will do some form of pull-ups (body in horizontal position, feet on footholds, pull-up and stabilize body on 1 arm, release, etc.).
That all fits well within my current 3-session-per-week routine and my family/job committments. It will also lead to a training plan that is very close to what nasher47 suggested.
Again, thanks to all who contributed!