Ickathu - way to be methodical and consistent in planning how to practice. i envy that self control and believe you will benefit hugely from it.
The ONLY function of Dominic or major system is to have something to fall back on when you can’t remember the card. Perfect association between the card and the image is the goal. It is easier to achieve with a system for most people, but you have succeeded in spite of not having that crutch. There may be good reasons to change some of your cards in the future, but that isn’t one 
Practice with immediate feedback is most effective, as a rule. Playing a song “ten times in a row, no matter what” is less effective than practicing until you can play it perfectly once or twice. Just going through the motions provides no feedback, so there is no guarantee that you will actually improve in a reasonable timespan. There is no specific goal that guarantees/requires improvement.
I fear that memorizing deck after deck is a variation of this form of bad
Practice. It’s not enough to say “I’m going to keep trying to do it faster.” That’s like saying “I’m going to score more
Points in this next basketball game”. It is a deceptively inane statement, and practicing shot after shot until they are perfect, practicing those shots scrom places he is less successful, is how he would REALLY improve his shooting effectively.
Can we brainstorm a few ideas to make practice more focused, and with direct feedback? I’ll start:
With a metronome, going relatively slowly, spread one card at a time. With the method of your choice, commit these to memory. When the deck is done, put the front card on the back of the deck and continue. To as far as you can without having to spend extra time on any given card. Speed up if it is too easy. try to break your high score. This helps with recognition speed, but not memory.
Memorize three decks in a row (with breaks in between) successfully. If you can’t recall a loci, go back to start. This works on memory, but not speed per se.
Memorize an entire deck, visualizing each new image WITHOUT letting the image of the previous loci leave your mind’s eye. This will be damn slow at first, you probably won’t even be able to do it once out of your first 16 tries, but I fear the man who masters this skill it’s functionally equivalent to spending twice as long visualizing each loci, and builds stronger associations betweenoci for easier recall. Don’t move on until you cleat see both loci at once, and mark your times for this exercise.
Using. Multiple decks, see how many loci you can fill up, without review, with perfect recall. Quit when you think you have reached your limit. Try to beat your top score.
Notice that each of these exercises has a very specific function. That is good for practice.
Well, I’m all out of ideas for now. Please contribute if you can think of anything.
Edit: Force yourself through a deck at high speed. Review and see how many you can remember. Deal the
Cards down in three piles of three cards each after removing the back card. Reassemble them so the image are the same but in a different order. Change palaces and do it again. Spread in groups of three rather than letting just the first card trigger the image. That helps you identify and master combinations of cards that may be tricky.
/protip:
If you spread in groups of three, notice when there are two of the same card, if they are all the same suit, or if they are all face cards. Your rote memory may save you.
Edit: just did some speed numbers for the first time in weeks, in three 2-minute trials, I got 56 then 62 then 68 digits with really clear recall, so I should probably go even faster. Last time I did numbers, 80 digits in 5 minutes was a “good score.” Just tried again , 50 (48) in one minute (2 errors)
I’m bringing this up because the last time I did digits, my cards had improved a lot in the 24 hours that I spent on just digits. Perhaps alternating them is the key? Has anyone systematically alternated between cards and digits?
