Overarticulation and hyperarticulation are terms used to describe this deliberate and exaggerated speech, often observed in infants or language learners. These terms can also apply to specific dialectal speech patterns. Wiktionary provides similar definitions for both terms:
exaggerated articulation
the hyperarticulation of child-directed speech
hyperarticulation/Wiktionary
excessive articulation
overarticulation/Wiktionary
These terms are also used in academic publications in the field of linguistics. For example:
The ability of speakers to exaggerate speech sounds ("hyperarticulation") has led to the theory that the targets themselves must be hyperspace hyperarticulated.
Whalen DH, Magen HS, Pouplier M, Kang AM, Iskarous K.
Vowel production and perception: hyperarticulation without a hyperspace effect. Lang Speech. 2004;47(Pt 2):155-74.
doi: 10.1177/00238309040470020301. PMID: 15581190.
When discussing language orthography, this phenomenon can be related to phonemic orthography. English, for instance, is highly non-phonemic compared to many languages but was mostly phonemic during the Middle English period. Languages such as Serbo-Croatian and Esperanto are often cited as examples of employing ideally phonemic orthography.
A phonemic orthography is an orthography (system for writing a language) in which the graphemes (written symbols) correspond consistently to the language's phonemes (the smallest units of speech that can differentiate words).
Wikipedia