How to configure pre-chosen static IPv4 addresses

I have more that 50 DHCP enabled devices on my LAN.

In my previous firewall (Untangle) I partitioned my local IPv4 network into a range managed manually (.1:.199) and a range managed via the DHCP server (.200:.253). I gave my many devices IPv4 addresses that, though served up via DHCP, were at least easy for me to remember and recognize.

Can I do anything similar with my Turris Omnia?

Yes, the same, what you write, you can do with Turris Omnia.
All necessary things you can find in Luci interface (Network -> Interfaces -> LAN -> DHCP for setting range of IP addresses managed automatically by DHCP) and other addresses out of this range you can set manually.
Or you can set some of the addresses from DHCP range as a static (Network -> DHCP and DNS).