"I know that "ia" is a spoken form of "iria"
No, not always. Even though "ia" is often substituted for "iria" in conditional clauses, as in "se você me convidasse, eu iria/ia" (futuro do pretérito), "ia" is the current form of "ir" in the "pretérito imperfeito do indicativo" (quando jovem, eu ia ao cinema aos domingos) and it isn't a substitute for "iria" here.
"Eu já tinha estudado esse assunto então a aula foi meio fácil" would have the more formal "Eu estudara esse assunto..."
Hardly anyone uses the "pretérito mais que perfeito" in the spoken language these days. Even in the written language it sounds dated.
"Does the tense represented by estudar in the phrase "Eu ia estudar mas eu tava com preguiça então só assisti netflix" have a more formal equivalent?"
In the above example, "ia" is "verbo ir" in the "pretérito imperfeito do indicativo" and cannot be replaced by "iria".
See the examples:
- Eu ia/iria com você caso não estivesse chovendo. (futuro do pretérito, and Brazilians often substitute "ia" for "iria")
- Eu ia pedir o carro emprestado mas mudei de ideia. (pretérito imperfeito, and you can't use "iria" here)
To make your sentence sound more formal, I would use "pretendia" (verbo pretender) or "tinha/havia planejado" instead of "ia" and would use "estava" instead of the outright informal "tava".
"Eu /pretendia/tinha planejado/ estudar mas estava com preguiça, então só assisti netflix."
The above sentence is not in the conditional mood and you can't, therefore, use "iria".
In a nutshell: "ia" is an informal substitute for "iria" in the conditional mood ("futuro do pretérito) If your verb tense is "pretérito imperfeito", however, there is only one form: "ia"