You can start with a step-down module like this one.
I've used it and it works very good, I don't think you have to worry about noise emission.
Later, if you make your own PCB, you can integrate the circuit on it, the chip used is MP2315.

Not only it wouldn't make much sense selling the modules with some preprogrammed demo application (wich will be erased by the user anyway), the price (time) to flash the application on milions of modules would make them much more expensive.

Use switching instead of linear regulator

You are building a commercial project and have to ask about how to power it !?

All WROOM and WROWER modules I've ever bought came with empty (erased) Flash, no firmware loaded.
Some sellers or board manufacturers may preload some firmware, but this should not be the case with bare WROOM/WROVER modules, no metter if it is 4,8 or 16MB.

I've used 14 MB SPIFFS (on WROVER with 16MB Flash) without issues.

This is definitely not normal.
Opening the file and reading should be quite fast (< 1ms per 4K block). Writing can be slow, as the sector has to be erased and that can take ~100 ms.

ESP-WROOM32 should have only 5 uA current in deepsleep.
How are you powering it?

As the value range for CONFIG_ESP32_PHY_MAX_WIFI_TX_POWER in menuconfig is 0 - 20, I can only assume the range for esp_wifi_set_max_tx_power() is the same.
It is indeed not clear from the function description and the function source is not available.

What solar charge controller IC do you use? No special solar charge controller IC is used, just MP2315 switching regulator providing 6V to the TP5000 battery charger setup for LiFePO4 charging and some additional circuit to disconnect the panel on low voltage. Any panel (or other power source) 6V -...

Isn't it much easier for the client to use his smartphone to manage the email, or connect his PC to the smartphone acting as hotspot (adding the USB wifi adapter shouldn't be the problem)?