Some more thoughts on this:
In order to combat the small hand size, and while still keeping the relatively slow pace a three card hand would impose, each player draws two cards at the beginning of their turn. The player taking the very first turn only draws one card on this first turn.
The sub-alignment idea is not the greatest, as it would be a steep learning curve, even if the game was digital in which case there could be dedicated ways to keep track of the sub-alignments, which is to say nothing of wanting to potentially play the game physically.
Instead, I'm thinking of potentially having a Celestial Guidance card. Essentially, the idea would be that, at the start of your turn (mandatory on turn 1), you can set one card from your hand as your Celestial guidance. By using card as cost, they generate Mana equal to their own Mana Value (the number of moons, suns or starts in the bottom left of the cards) of the colour of you Celestial Guidance's Mana Identity (If it's a sun, moon or star card).
Taking these two cards as example, the situation is as follows: Pegasus is set as my Celestial Guidance, and I use Fountain of Polaris as Mana cost. Fountain of Polaris' Mana cost, as indicated in the lower left, is 4, and Pegasus, as indicated above the card art, is a Sun card. Hence, using Fountain of Polaris for cost would generate 4 Sun mana.
Celestial Guidance cards could be discarded and replaced with another at the beginning of each turn, in order to not lock decks into a single colour identity. Ideally, some cards would have effects which interact with being the Celestial Guidance card, in order to not make it feel like a dead card you don't really get to play.
Another point I'm considering is the number at the bottom right of the cards. While I originally interpreted them as Attack and Life values (a very Magic the Gathering way to look at it), this doesn't make much sense in a system not about direct with another player. It could be interesting to have those values represent different 'states of being' for the cards. This would make each card, essentially, two cards in one, with one version being the regular state and the other being the Eclipse state (to keep with the celestial bodies flavour.) The left number would be a value in the regular state, and the right would be a value in the Eclipse state. (Which value that is, I'm not entirely certain yet.) Cards would also have different effects in regular and Eclipse state.
This still leaves me with yet unsolved problems, amongst them Mana cost. In the absence of other ways to generate Mana, I have defaulted to using the cards as a way to pay for costs. This would cause an issue, I think, as even with a way to recycle discarded cards, (which I will inevitably have to work into this version again to combat the small deck sizes,) having a bunch of cards in your deck meant as, essentially, discard fodder, isn't very fun. Which means I should either design the discard system around the fact that discarded cards need to factor into a player's game plan and be part of their strategy, or come up with another way to generate mana to pay for cost.
The other big problem, as it stands, is interactivity. As it stands, the game would be mostly just two people doing their own stuff, on their own turn, with no interactivity between them. I need to come up with ways for the players to have opportunities to disrupt or counter the other player's moves.
One idea I considered would be to make Shiny Chariot TCG and asymetrical card game. My point of reference was specfically Genius Invokation TCG, the Genshin Impact card game. In the in-game version, many duels are challenges featuring the game's monsters and environments, mirroring the broader situation of characters fighting monsters in the wild. This, however, would mean having to use the cards again, in a dedicated alternate version with their own effects and rules, and asking players to build not one but two decks at minimum to play. While it's not something I want to dismiss outright, I feel like this could risk making the game feel cluttered and bloated, for lack of a better word, not to mention making the learning curve even steeper.