It's a custom motherboard but not a custom chipset. The chipset comes from AMD.
> I'd also be keen to swap the Ryzen 3700 for the newer series chip .. but have no clue whether that is possible on that motherboard
If the chipset is A520, B550 or X570, you have Ryzen 5000 / Zen 3 support, though it probably requires a BIOS update. You might also have Zen 3 support on B450, B550A, X470 [0]. Check the BIOS release notes for your machine.
> I made the mistake of not getting the system watercooled
Most water cooled systems only water cool the CPU, which in your case (a 65W TDP CPU) is absurd. Good air coolers match the performance of twice as expensive AIOs [1] and have far less potential for breakage.
> Also, can I say AMDs numbering makes no sense to me personally.
The first digit is the generation. 1000 series -> Zen 1 architecture, 2000 series -> Zen+, 3000 series -> Zen 2, 5000 series -> zen 3. The higher the number after that, the better the CPU is within the generation (more cores/cache, higher frequency).
[0] https://www.anandtech.com/show/15807/amd-to-support-zen-3-an...