Both Initially I was planning to make this podcast ... and Initially I planned to make this podcast ... are grammatical. The reason that the speaker uses the progressive aspect is because he or she recalls or conceives of the planning as being of some duration.
It is worth citing Michael Lewis in The English Verb (p42) here because what he says is fundamental to an understanding of the speaker's choice of tense or aspect in English.
We have already seen pairs of sentences both of which follow the rules
of grammar as fact - in other words they are "correct" standard
English. ... The differences (in meaning) are based on a choice made
by the speaker at the moment the language was used. The importance of
this idea is impossible to over-estimate. The speaker's understanding
of the situation, intentions, and interpretation of the facts are
central to the language the speaker uses.
Later Lewis reminds us that aspects of the verb, such as the progressive form:
... do not refer to real time but to psychological time - to the
speaker's perception of the temporal quality of the event. ... The
essential characteristic of (be) + ing forms is that the speaker
uses (be) + ing if, at the moment of speaking, s(he) conceptualises
the action as existing for a limited period of time.
This contrasts to the simple form, which:
... expresses the speaker's view of the event as a complete, unitary
On this basis it is not helpful to think that 'during /period / time of planning is always followed by progressive' (OP's comment), but to use the progressive whenever you wish to convey the durative nature of an event.