Is it better to use thin-set or self-leveling concrete on top of a shower pan, mud-bed to improve the slope and make it smoother for final tile layer?

Issue:
I am looking to improve the smoothness of my my concrete mud-bed before I lay my thin-set and tile. The mud-bed has dried, but there are some dips and divots that make it difficult for me to make the tile job smooth while maintaining the slope. I keep finding myself left with imperfect spots in the tile floor where water pools and that I have to push towards the drain with my foot (for example, one side of the drain is level but the other side dips below the drain and is not flesh). I have ripped the tile and thin-set up twice now. This time I am considering adding about another half-inch layer to (1) smooth the concrete surface and for my thin-set and tile job, and (2) improve the slope (if I can)

Question:
(1) Should I apply a 1/2 inch layer of self leveler on the dried concrete bed to accomplish this? Or
(2) Should I apply a 1/2 inch layer of thin-set to smooth the surface, let it cure, and then lay my thin-set, score and tile? Or
(3) Is this issue simply a matter of being more skillful in laying my tile on the concrete bed; or
(4) is ripping out the concrete bed, and starting fresh the only way to fix?

Background and steps of installation so far:
I am putting the final touches on a small walk in shower with a curb.

I had some professionals frame, drywall (cement board), water-proof, etc. I shopped around for tile-laying prices and decided that I’m better off trying myself, even if I wind up having to rip some tile and re-lay. I have the walls up fine. The floor, of course, has given me minor issues. I’m at the finish line though and am determined to finish it off.

First Try:
First, the learning experiences came when I used pre-mixed thin-set for the shower floor tile. The first time I payed the shower floor I used a mosaic penny tile. The slope seems okay because the water drained fine. There were a few areas, however, where water sat due to uneven tile. Nevertheless, after about two months, we noticed the floor became squishy in some areas where the water gathered and sat. Since I didn’t like how the white penny tile looked against my blue walls, I was happy to rip it up. The mortar and tile came up very easy (should have been my first clue), and the cement bed was unscathed.

Potential Problems:
I tried to diagnose the issue, and concluded that the problem was either (1) because there were spots where dried thin set poked through the grout, causing a thinner layer of grout than need be, and allowing water to seep through, (2) the uneven areas of the tile floor caused the grout to break and water to seep through; or (3) perhaps I even covered the weep holes which blocked water from reaching the drain.

Second try:
I determined the weep holes were not blocked because the water drained when i let water get around the edges after I removed the tile. So I took made better tile cuts (and used a better looking mosaic tile) and it turned out great, and less uneven (but still had some uneven areas around the drain). Over time, the tile
Shifted because, yet again, the floor became mushy. The grout popped up and water got underneath the tile. So, I read the back of the pre-mix thin-set label, and of course, it says to not use on floors and areas with heavy water immersion.

Upcoming third attempt:
This time, I am determined to get the floor even and avoid pools of water. I bought a different thin-set to use (kerabond T by mapei mixed with keraply) and a different grout (ultra color plus FA by mapei), that I am using for a mosaic octagon tile.

Any recommendations on how to fix these problems would help! Also, if anyone spotted any other potential issues that might be the cause to mushy tiles, please let me know. If there are deeper rooted issues that I am not noticing, might be time to bring in professional help.

Thanks everyone!