In your first example, because "estoy esperando" already introduces "yo", it's better to use the infinitive: "antes de empezar a cocinar el pollo".
You can say: "La freidora de aire tiene que enfriarse antes de que (yo) empiece a cocinar el pollo". Here, there is no "yo" involved in the previous verb, so I can use a conjugated verb for "yo" instead of the infinitive ["que (yo) cocine"].
And the "yo" is not necessary, unless the speaker wants to make an emphasis on who is cooking.
In the second example is the contrary:
Although it's more "economical" to say "antes de irme", and people do speak like this, the best is to say "antes de que me vaya".
When the previous verb is already about the same person (in this case "yo"), then the infinitive is better.
Since "necesitas algo" refers to a different person ("tú"), the second verb should be conjugated to talk about the other person involved ("yo").
- ¿Te ayudo en algo antes de irme? -> Here, "yo" is in both verbs of the sentence, so the infinitive is the best choice.
To me, your sentences aren't really wrong, becaue many people do say "¿te ayudo en algo antes de que me vaya?" and "¿necesitas algo antes de irme?", but the preferred "rule" is that same person needs an infinitive and two subjects need a conjugated verb.