Hi Mercurian. I'm not sure how you prefer to read, so the following may be of no interest. I'm sure you already do some of this, but it also includes astrology, hermetic Qabalah, etc. Feel free to skip this if it's not your thing.
In linking cards, I look for shared colors and symbols, whether the figure in one card is facing another, whether it's more or less crowded on a card, etc.
I look at where the suits and associated elements might be the same or how they might combine with one another. (I don't read with elemental dignities, but that could be part of it.) And the suit(s) that
aren't in the spread - what energies are either irrelevant or missing?
I look at the numbers and whether they're close to one another in sequence or far away.
Then I notice the correspondences that went into the creation of the RWS and Thoth.
For example in the 4 of Pentacles and 6 of Cups --
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The RWS 6 turns its back on the 4. In the RWS the disks were held tightly while the cup is given to another.
The yellow of the RWS pentacles becomes the yellow all over the 6 of Cups as though through connecting, we paradoxically have more instead of less. In the Thoth the flat, gold background of the earth becomes flowing, gold lines and water - changing from stillness to movement.
On the Thoth you see the ⊙ symbols that show that both cards are associated with decans ruled by the Sun (more yellow/gold). Sixes are also
associated with the Sun. So the influence of the positive and rational Sun is increasing. (If the question were "What should I avoid?" then the Sun's potential for excessive ego would come to mind.)
We go from Capricorn (4P) at the beginning of winter when you traditionally needed to store what you had, to Scorpio (6C) in mid autumn when the harvest has just come in. So we could say there's less pressure on us in the 6 than there was in the 4.
Both cards are numbered in the middle of the suit rather than a beginning/end. The fours to me are temporarily stable but eventually hit the challenging fives before reaching equilibrium in the sixes. In this pair, though, we have the 4 and the 6 without the hardship of a 5 between, which bodes well for an easier transition. There's no fiery Wands card that could bring to mind a lot of energy expended, or a Swords card that could point to choices and planning.
(I might also consider the Hermit as a connection between the two cards, because in the Qabalistic correspondences for the RWS and Thoth, the Hermit represents the
path between the fours and the sixes. I'd ask how the Hermit might be the force that gets me from the 4P to the 6C. I'd look at his lamp, like he's got a little bit of the sun he carries with him.)