Large Alpha/numeric can be good. 7 segment is awful limiting, but it good to understand the goals. What are you proposing for a power target, to inform backlighting suggestions? EL can be nice, but rgb LEDs are alot more flexible and I believe you said decided. eInks are interesting, but have funny characteristics and sometimes high write voltages for some of the technologies. Understanding the power goals is a good way to eliminate many options.
So we are talking a large limited digit display primarily for Numbers? Is it limited to 4 digits, or 5?
Time only requires 4, but many other uses do better with a 5th or 6th digit. This also gives a digit for the “:” units specifier if possible. I will confess I really prefer displays that are clearer than a simple 7 digits as the letters/numbers are mangled to really be displayed so I would encourage doing a little better than just straight segments. Big blocky displays are less interesting than they were when we could first build them.