Platform Choice: Mojo V3 or Other?

In my experience, working with any FPGA is pretty much the same. There are some differences if you start doing more advanced stuff and utilizing some specialized resources (like SerDes or special clocking resources). However, most FPGAs have equivalents of whatever you need.

I would recommend not using the Mojo any more just because ISE is so poorly supported by Xilinx. This is true for any Spartan 3 board you may find. Get something supported by Vivado which is substantially better.

Using Lucid and Alchitry Labs or Verilog and the vendor tools are a personal choice. Lucid isn’t that different from Verilog but is way more straight forward for most things. I think it is a great place to start and transitioning later isn’t hard if you want/need to.