Currently, MT defaults are managed via config/mt-defaults.wikimedia.yaml, which is difficult to maintain with more and more language pair additions and likely error prone. Split it or manage it via some other way to make sure it is readable and manageable.
There are 855 lines of coniguration that maps language pairs to 'source- mt' https://github.com/wikimedia/mediawiki-services-cxserver/blob/master/config/mt-defaults.wikimedia.yaml
I see no reason to have source-mt as default if there is an MT engine(If not present source-mt become default anyway). The UI allows to easily choose a preferred one and persist that option.