Once the technique for correctly positioning the feet is learned, we could also discuss different settings according to the various repertoires you intend to tackle; the main problem lies in the independence that must be achieved relative to the hands, exactly like when on the piano one must overcome the difficulty of making the two hands play autonomously yet coordinated.
Therefore, instead of a specific method for the pedalboard (which do exist), I suggest you take an organ method, specifically: LO STUDIO DELL'ORGANO - NUOVO METODO PER L'ALLIEVO ORGANISTA, by Molfino Delmonte.
There is also that of Germani, but frankly from a certain point onwards it perhaps becomes unnecessarily difficult and it's better to learn the basics directly on the pieces.
Obviously, with a teacher it's a whole different thing, also because alone it is difficult to recognize and correct defects, but by following the indicated method from beginning to end several times, one achieves good results, obviously starting from a certain level of ability on the keyboards (let's say at least a level like 5-6 years of piano, old system).
That said, however, I immediately disagree with you regarding the discussion on piano technique.
On the organ, especially with mechanical action, the technique is different and one cannot play a piece as one would approach it at the piano because one must compensate for the lesser (note that I say lesser and not absence) possibility of managing intensity and sound attack, with a phrasing different from that of the piano.
Last point: whether the two instruments are incompatible.
My experience leads me to say that excellent organists are often excellent harpsichordists, but they are rarely pianists who can compete with those who are exclusively concert pianists.
Excellent pianists, sometimes I have heard them perform on the organ, even with dexterity on the pedalboards, pieces played very well indeed, and in this regard the instrument helps to minimize the differences between a concert organist and a pianist.
In general, it depends on what level you are at, how solid your preparation is, how much time you study, and what goals you set for yourself on both one instrument and the other.
Anyway, what differentiates an organist from a pianist is also a different approach to the instrument.
An organist must get used to playing on different instruments in terms of timbre, size, type of keyboards and pedalboards, and therefore also adapt the repertoires; a pianist will typically always find 88 keys of standard size, even if the piano underneath might be dull, poor quality, etc.
Finally, an organist, especially a liturgical one, must know how to improvise, something that is no longer required of a modern pianist.
These are just some aspects and there would be much more to say.
bye
