But the main question is: Have you found cheap maker boards with such mcu's like STM32L4S5 or STM32L4R5?
For me it is not the main question. Simply because there is no board that will fit all my needs regarding peripherials, size and shape. So for every project I need to design my own board, so it does not matter if there is a cheap maker board or not. Besides I do not recall any nrf52 maker boards that are cheap. Certainly You cannot call a maker board those small smd modules from aliexpress - they all required some kind of motherboard.