The typical max7219-clone driven single-color 8x8 LED modules you see at Banggood/eBay/Aliexpress, actually only light one 8-LED row at a time. The current through each LED is controlled by a single 9.5k-47k resistor connected to MAX7219 ISET pin (pin 18, next to positive input pin), setting maximum peak current per LED to 100*VCC/R, with maximum limited by the driver IC itself to 40mA or less.
Thus, these will have a varying current draw even when displaying static LEDs, at up to 320mA per module, assuming the module does not try to overdrive the LEDs too much. For the ubiquitous 4-in-one clone module, 32×8 matrix, powered from 5V, that corresponds to 1.5A or so (1.3A + losses) draw at 5V with all LEDs lit. You'll want to use a switchmode supply that can provide at least 1.5A at 5V, preferably more; and preferably an automotive one considering the spiky 12V supply in a car.
LCSC and JLCPCB (for assembly at JLCPCB) currently sell the
Hgsemi MAX7219 IC (non-English
datasheet) for under 1.4€ apiece, so you could design your own board for your preferred 8×8
common cathode LED modules. You only need the current-setting resistor and a couple of bypass capacitors per 8×8 module. It is often easier to align the LED modules inside a frame, than use the PCBs to align the LED modules.
Putting a tinted/colored plastic sheet of the same color as the LEDs over the display will make it easier to read. You can also consider getting these in the kit form for easier control of the Rset resistor (10k - 47k, lower resistance yields higher per-LED current).
For WS2812B, each LED includes their own current control, maxing out at about 60mA per LED, and generating 90mW of waste heat when run from 5V. Indeed, lighting 256 such LEDs (a full 32×8 matrix) would consume over 15A. You can reduce the waste heat by running them from a lower voltage, about 3.8V to 4.2V, reducing the waste heat generated. The current draw is unaffected, but the total power draw is reduced. I recommend using 74LVC1T45, 74LVC2T45, 74LVC8T245, 74LVC16T245, or TXU0104 for the level shifters (from 3.3V logic levels to 3.8V-5V you use for the LEDs).
Some kind of diffuser is often used that passes 80%-90% of the light through, but scatters it.
JLCPCB currently sells Xinglight XL-5050RGBC-WS2812B for about $9.5 per 300 when assembled (C2843785), so designing your own WS2812B matrix modules as single-sided aluminium-core PCBs (also a special offer right now 5 pcs 100mm×100mm for just a couple of dollars) would be a possibility, with final assembly on top of an extruded aluminium heatsink. Even if it draws only say 8A at 4V, that's still almost 32W of heat generated. Note that for reproducing Western letters, an odd number of rows is better than an easier number of rows: 7 or 9 is much better than 8. As typical letters are then 5 LEDs wide, you can count 5+1 (letter spacing) = 6 LEDs per letter. Add occasional bypass capacitors between supply and ground for added stability. You can chain up to 1024 LEDs in one data chain before you need to consider splitting them into multiple parallel chains. I would consider creating 11×11-LED panels with vertical data chains (leftmost column first, then next column, all either bottom to top or top to bottom), as then each letter would be consecutive in memory even when displayed (similar to small 128×32 OLED displays), with slightly less than half the LED spacing on the sides, allowing continuous horizontal display by chaining modules.
All that said, it just might be more cost effective to use a small Linux SBC and an IPS TFT display instead; or perhaps a (hacked) digital photo frame.
An e-ink display would be even better, but large ones are pretty expensive. BuyDisplay 4.2" (90mm×90mm, 85mm×61mm 400×300 active area) e-paper display modules cost about $23 apiece, and using multiple modules you'd have a small gap between modules, but would be daylight-readable. Power would only be drawn when changing the displayed text, and it's black-and-white only. Waveshare has 5.8" (152mm×52mm, 139mm×48mm 792×272 active area) e-paper modules for about $32 apiece, with cheapest BW 13.3" (286mm×212mm, 275mm×195mm 960×680 active area) SPI module costing about $150.