You indicated "hints" were OK, so here's a
I suspect that there is a way to get 1.8V out of the RPi 4B - but this is only a suspicion. I only read of it a day or two ago, and I've not tried it, nor explored the idea in any detail. The basis for my suspicion may be found in the CM4 datasheet. Reference para 3.3 Electrical Specification:
Please note that Vref is the GPIO bank voltage which must be tied to either 3.3V or 1.8v rail.
Note that the CM4 module is a different platform than the RPi 4B. However, it uses the very same BCM2711 SoC as the RPi 4B, and the "official documentation" on GPIO specifically refers us to the CM4 Datasheet for details on the GPIO voltage specifications. Or, perhaps it may be necessary to purchase a CM4 and associated CM4 IO board to get at this?
Of course, it is entirely possible that I've not properly translated the "official documentation" into proper English :) If no one here in RPi SE suggests anything more substantial, you might post an "Issue" on the RPi firmware GitHub site - some of the staff are very helpful.
If it turns out that my
hint-but-not-an-answer is rubbish, then I have one other answer that is very likely more substantial:
A real answer:
I have wasted enormous amounts of time attempting to configure RPi hardware using the tools provided by The Foundation -
/boot/config.txt, rpi-eeprom bootloader configuration and so forth. I believe this is due in large part to my background - I am more comfortable with systems ruled by physical laws than man-made (e.g. software) laws. I say all of this only to underscore that a hardware solution may be a more time-efficient solution.
A potentially straightforward solution to your problem may be to use a level-shifter. You're likely familiar with these devices, but here's a brief intro written by TI. This link may suggest some specific TI part numbers that meet your specifications.
One final point: The majority of ICs produced today are in surface-mount packages, and these are not particularly easy-to-use for hobbyists. If you have the inclination, and basic soldering skills, there are generic "breakout boards" for many common SM packages, and if not several vendors will sell you a customised breakout board tailored to a specific IC.