The idea to upgrade just this one step from Sierra 10.12 to HIGH Sierra 10.13 comes from me having 32 Bit Apps that I wouldn’t want to loose and I thought that High Sierra is the LAST macOS to support ALL those 32 Bit Apps… ?! I had read somewhere that Mojave does NOT support ALL 32 Bit Apps… but I could have mixed that up…? **
I still did not dare to upgrade and am a bit upset now because I really would like to try out Blocs 4… sigh… 
All my old 32-bit apps continue to work with Mojave, though there are not many at this stage.
No doubt there are some apps that have difficulties with Mojave just like any other OS. I would suggest contacting the relevant developers if you have any concerns. Right now I am doing 95% of my work on the new mini running Big Sur, however Mojave is still perfectly usable.
I see…
thank you @Flashman 
One problem is, that one 32Bit app is discontinued, so no hope for a future adjustment to 64 bit…
Would you see any advantage or disadvantage whatsoever when upgrading first to High Sierra and then after that to Mojave?
I guess you would go right to Mojave, right? 
I would go straight to Mojave with a clean install after making a backup, then only install what you really need. If you are installing one OS upgrade after another it is more likely to cause problems, whereas a clean install of the last point update should be very stable. Chances are it will also have less problems with your apps.
Really it is your decision though how you prefer to go. In my case I had a lot of problems with High Sierra, which had just introduced the APFS file system. A clean install of Mojave proved much better.
I guess, the „CLEAN install“ are the magic words here. I do not dare to tell you what ancient stuff I found. 
Actually I always went on doing one new OS on the next…
archeology !!!
Would you choose an SSD or a (cheaper) hard drive for the backup (from which I would actually like to work from if necessary because of some apps). ?
An SSD is always better, but give some thought to how much longer you want to stay with this computer rather than buying something newer.
The new M1 machines change everything and there may be some relatively new Macs on sale soon for much less money than you might expect. For example, I spent the best part of £2000 on a Mac mini earlier this year that is slower than the new M1 costing half as much, so it would be hard to sell for even half of what I paid only six months ago.
That’s OK because I use it for work and it paid for itself in the first month. At this point I’ll keep it and it will become my backup in a couple years time. It makes the used market interesting for buyers though if they are happy to buy an Intel machine and the new M1 machines are competitively priced as well if you can afford one.