On which surface should the shower tray be placed?

Good day, I am new to this forum and I hope to get some helpful tips.

Since I need to renew the floor tiles in our bathroom, I also wanted to replace the old shower at the same time. The desire was, of course, a walk-in shower, although I was aware that a certain depth is required for the approximately 2-degree slope. Under the old shower tray was a kind of concrete foundation (?), on which the shower tray was placed. When I painstakingly chiseled away the entire foundation, I found not only the drain but also the water pipes for the shower and the adjacent sink (there are no plans so I couldn't check this beforehand).

Now my question (and photo attached):
The idea of the walk-in shower is obviously off the table, so I will just install a simple but lower shower tray. I am also familiar with all the work steps like sealing tape, waterproofing with liquid membrane, etc. But what could I put the shower tray on if I do not want to fill the whole "hole" with concrete again? Could I also glue construction panels (for sanitary areas) and place the tray on them? I want to implement the whole thing as simply and quickly as possible.

I look forward to any suggestions.

Best regards, Florian

The (probably?) usual shower tray feet don't work?

How are they mounted to the shower tray? I don't know if it will work with the pipework, or are the feet adjustable?

Unfortunately, I can't tell you that. I only saw this or a similar construction when our installer installed the new tub. I assume that the frame can be adjusted somehow – the pipe distances are different for every shower. Unfortunately, you will have to check with a professional.

But what could I put the shower tray on?

There are shower tray supports for that. So little feet and angle profiles for the wall. This allows you to place the shower tray rock solidly (you can even put the feet, if you want to go all out, into little spots of screed) and align it perfectly.

Depending on the height and type of shower tray, there are different variants. The feet can sometimes also be placed freely. Either the shower tray is just laid on or the feet are screwed to it, for example

or are the feet adjustable?

They should be. That is what they are for, to compensate for uneven surfaces.