I think key signature is right - but one chord does not fit?

[lfm]

So I would say Ab major with 4 flat accidentals are the best option as key signature. Notes fit, all chords fit but one.

Chord sequence F, Db, Eb, Ab, Bbminor

But F is the VI in Ab major, and should be F minor not major.
- at least according to basic chord analysis
- but that does not sound right here

So F major is the riff based on and it sounds right picking notes from that. F minor sounds off. Wrong tonality.

Trying to get notation and things correct and to grasp it all from theoretical point.

In notation do you put 4 flat accidentals as key signature and then F as chord, even it should be minor.
- or a Fm+3 or something, as a raised 3rd?

There are dim chords, 1, b3 and b5.

Is there such a thing as a raised 3rd to fit in the key signature?
- it's just a F major in this case, but to fit key, I mean
- some altered minor instead

dim7 is an interesting chord in how it works

Cdim7 a C, Eb, Gb, A and can be called by either note as base note
- Ebdim7
- Gbdim7
- Adim7

4 stacked minor 3rd intervals.

then you can pick one that fits key signature

Chorus is in C major, so nothing funny there.

[DocAtlas]

So I would say Ab major with 4 flat accidentals are the best option as key signature. Notes fit, all chords fit but one.

Chord sequence F, Db, Eb, Ab, Bbminor

But F is the VI in Ab major, and should be F minor not major.
- at least according to basic chord analysis
- but that does not sound right here

So F major is the riff based on and it sounds right picking notes from that. F minor sounds off. Wrong tonality.

Trying to get notation and things correct and to grasp it all from theoretical point.

In notation do you put 4 flat accidentals as key signature and then F as chord, even it should be minor.
- or a Fm+3 or something, as a raised 3rd?

There are dim chords, 1, b3 and b5.

Is there such a thing as a raised 3rd to fit in the key signature?
- it's just a F major in this case, but to fit key, I mean
- some altered minor instead

dim7 is an interesting chord in how it works

Cdim7 a C, Eb, Gb, A and can be called by either note as base note
- Ebdim7
- Gbdim7
- Adim7

4 stacked minor 3rd intervals.

then you can pick one that fits key signature

Chorus is in C major, so nothing funny there.

If the verse is in Ab, use that key signature. Your chords (it sounds like you're writing them above the staff) don't have to be diatonic to the key. If you want an F instead of F- in the key of Ab, just write it that way, You don't raise the flatted 3rd in a minor chord if you want a major chord; just indicate the major chord. If you're writing the chord out on a staff, then make sure to use a natural sign next to the 3rd if the key signature calls for it to be flatted.

As for the diminished 7th chords, it's true that they are simply stacked minor 3rds, but technically, the 4th note in Cdim7 should be written as a Bbb, not an A. In 12 TET, it doesn't make any difference in the sound, but writing that note as an A makes it some sort of 6th chord, not a 7th. If the chords are being written on a staff, it's visually better to write it at C Eb Gb Bbb rather than C Eb Gb A. It won't make any difference to the listener, so use whichever way works best for you.

Bottom line is, you can always use chords from any key or scale, no matter what key you're currently in. If you like it, it's good.

[lfm]

Thanks for clearing things up for me.