Up to now I can select that the next episode to download can be either the newest one or oldest one.

I suggest to introduce a "catch up" mode.

Therefore I would introduce an Abo date. this is the date when I started the Abo.

I would suggest such a function.

next epidode to download (abodate)

if there is at least one episode newer than Abodate
then take the newest episode
else
take the oldest episode
end

With such a function I would chronologically catch up with the older episodes but also hear new episodes close to the release date.