Modern CPUs are more difficult to program in assembly.
The simplicity of RISC-V is illusory. Because it lacks many features of normal ISAs, like ARM or Intel/AMD x86-64, writing programs that are both efficient and robust, i.e. which handle safely any errors, is quite difficult in assembly language.
For a simpler programming in assembly language it is hard to beat DEC PDP-11 and Motorola 68000 derivatives.
However those are no longer directly useful in practice. For something useful, the best would be to learn assembly programming using a development board for some Cortex-M microcontroller, preferably a less ancient core, e.g. with Cortex-M23 or Cortex-M33 or Cortex-M55 or Cortex-M85, i.e. cores implementing the Armv8-M ISA (the latter 2 also implement the Helium vector extension).
Probably some development board with a microcontroller using Cortex-M33 would be the easiest to find and it should cost no more than $20 to $30. I would not recommend for learning today any of the development boards with obsolete cores, like Cortex-M0+, Cortex-M3, Cortex-M4 or Cortex-M7, even if those boards can be found e.g. at $10 or even less.
Such a development board can be connected to any PC with a USB cable. All the development tools are free (there are paid alternatives, but those are not better than the free GNU tools).
You can compile or assemble your program on the PC, then load and run it on the development board. You can have a serial console window connecting to your program, by using a serial port on the development board and a USB serial interface. All such development boards have LEDs and various connectors for peripherals, allowing you to see what your program does.
I think that learning to program in assembly such an Armv8-M microcontroller is more useful than learning something like 6502. Armv8-M is less quirky than 6502 or RISC-V and it is something that you can use for implementing some useful home device or even professionally.
Otherwise, the best is to learn the assembly language of your personal computer, e.g. x86-64 or Aarch64, but that is much more difficult than starting with a microcontroller development board from ST (e.g. a cheap STM32 Nucleo variant), NXP, Infineon, Renesas, Microchip, etc.