I stand by cryptomatte being overkill for this use case. Render off your proofs at 5 or 10% resolution, get approval from whoever needs to approve them, tweak as necessary, then render the final resolution image.
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It’s finally solved.
As I mentioned before, I am using 10 x 10 tiles to make rendering easier for Blender.
So while doing some test renders, I noticed that rendering without tiles but with 10% of the original size worked much better than rendering a single tile at 100%. Still wanting to use tiles, I checked ‘Crop to Render Border’ so the whole ‘Compositing’ (?) part really at the end only needs to be done for the rendered tile and not the entire (empty) image anymore.
This worked great as Blender finally continues after having rendered the first tile while keeping the used CPU and RAM pretty low. The whole ‘Compositing’ part now takes a few seconds instead of minutes (!) which always resulted in crashing Blender.
The only problem I still had was that the RAM went up after each rendered tile. After disabling the Denoiser, the RAM finally became stable and Blender used much less render time.
I further noticed that it makes a great difference if you use a ‘packed’ .blend file. As soon as I made all textures external instead of ‘packed’, the render times became even better and the used RAM while rendering dropped from 3600 MB to 500 MB.
Thanks for your help!
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I know this is solved, but it is always wise to have a talk with the printer guys who are going to create the banner.
I’ve made large banners like this in the past, but never went above 100dpi. Why? Because these things normally are way way back for you to look at. Also, modern printers are very good in interpolating pixels for large prints. And my PS version started to complain about the max file size 
300 dpi is nice for regular print, for stuff like this, not really.
You found out the software and hardware limitations dealing with such large files, and it’s not necessary. Remember that everything needs to be stored in RAM before saving it, and 3d renderers are notorious for being bad at this for huge renders.
It also help if you can render the file from the command prompt, to avoid the UI overhead.
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Thank you @RobWu for commenting. 
You’re right, I know the banner will be visible from a great distance, but I always try to keep as much quality / DPI as possible in my designs. So if I hadn’t been able to solve this issue, I probably would have lowered the resolution to 72 DPI as you suggested (BTW, I used 150 for this scene, not 300 - which indeed would be overkill )
I am also already using Terminal since it saves me lots of time, but thanks for your advice! Much appreciated!
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