[Disclaimer: just my personal opinion]
I’d bet on postmarketOS, but the T9 friendly UI is not there yet.
It is available for the KaiOS phone that I own, though I haven’t tried it yet. Nokia 2720 Flip (nokia-beatles) - postmarketOS Wiki
Maybe a compatibility layer to run KaiOS/boot2gecko web apps on PostmarketOS help with app availability a bit.
Or just get a used second hand Android with custom ROM and custom minimal launcher that removes distractions. 
Pinephone is not worth the money unless you want to to tinker, it is way too slow IMO (I have one). I would look for used devices instead like an old google pixel, which has good custom ROM support and you get a much better specs for half the price. 
If they only have this phone, then they can not install a custom ROM, so you would need to install the ROM beforehand and that work costs extra money. 
The reality of those internet enabled phones is that most of the time, they only have the proprietary internet pre-installed: like facebook, tiktok, youtube, google maps.
Making a delta chat version that is an installable app for KaiOS would require atleast chatmail core compiled to wasm or rewritten in javascript, since you can’t run native apps on KaiOS without rooting the device.
Whatsapp had an exception for the use of their libsignal encryption library, but it costs a lot of money to get your app pre-installed onto new devices, so unlikely they would cooperate for free with an open source project. (BTW WhatsApp gave up and is no longer available on KaiOS AFAIK)
Since Delta Chat is offline first and stores all messages you need at-least some gigs of device storage + 14-60mb ram and a decent CPU for fast crypto. If the phone is too cheap it may not fit the requirements. (see What would be needed for a Standalone web version without a server component? for what’s needed to run chatmail core under WebAssembly)
If this is about making DC popular, then we could think about trying the Delta Chat for Cloud Phone idea, but that kinda defeats the purpose of end to end encryption, since the app would run on some cloud server and would only be streamed.
I still think it would be nice to have a delta chat client for kaiOS phones, but I would be happy with the experiment an proving that it is possible on rooted phones. 
I don’t have any hope or expectations that it could change the world, based on my impression of the state of these “dumb” feature phones. 