Discourse is also open source and nothing is actually preventing folks from making a version for older apps or even using older versions if they really must. someone even made a terminal version. also this discussion about apple native apps may also be of interest. good luck 
my ipad 6 was released 2 years before your iphone 12 mini and it works perfectly fine for Discourse. speed is most definitely not an issue, especially in mobile mode. i’ve used it for Discourse admin work coding and theme development. have you considered that some of your mobile speed issues may be network related?