Here are the results of two tests to decrease the size of the /boot contents:
  1. moved out the g-zipped symvers and vmlinux - the usage of /boot goes back to 24%; upgrade to 15.2 still fails with an estimated usage of /boot of 116%
  2. in addition: dracut --hostonly --force - this results in only a marginal decrease of initrd from 1.417 to 1.416 MB, so I didn't even try to attempt the upgrade with that

So, in conclusion up to this point: there seem to be no easy way to solve this problem and increasing the size of the /boot partition on my system would be the path to go.

Thus, I would like to come back to mrmazda's suggestion to do this as
You should be able to shrink the sda2 lvm at its front, backup the content of sda1, delete and recreate sda1 at a larger size, then restore sda1 content, with plenty of room on /boot/ resulting.
Can you give me some additional advice on that? Maybe some of the following questions are somehow obvious to the expert, but I would like to learn about disk modification in this process.

From what I read, I would probably have to do this from a live CD or USB stick to have the partitions on the hard disk unmounted for such a process. Right?
Which tool would you recommend? I read about gparted, but could the Yast partitioner be used as well?
How would I shrink sda2 at its front (which probably means at the border between sda1 and sda2)? I seem to find information how to shrink at the end, but then I would have to move the contents of the file system somehow to the end of the partition. Is that the idea?
I am not sure, why sda1 needs to be deleted/recreated and consequently its content needs to be "restored" - is there no way to enlarge the file system in sda1 once there is enough free space at the right place?
Once there is "free space" at the front of sda2, this could be used to enlarge the size of sda1 by "recreating". If I would want to take the "free space" from sda3 rather than from sda2, would I need to do this precedure twice, i.a. shrink sda3, "move" sda2 and then enlarge sda1?