Well, basically, it's that. I have a DOS EXE file and I would like to disassemble it to Flat Assembler format source code. If I can do it directly in one go, that's great. If I need to first disassemble it to, say, TASM, and then convert that to Flat Assembler, as long as it works, it's OK too. But I would like the process to not require Windows, since I'm a Linux user. If no other choice, I'll have to install Wine to do it once and then I have my source and won't need it again, but it'd be great to not need to do that.

What do you guys recommend for the disassembly?
Thanks

objdump -d -M intel file.exe

The closest output format from some of the most popular disassemblers, is probably NASM.

Then with a bit of scripting magic you could do post-processing of the NASM source.

https://github.com/gdabah/distorm/releases DISTORM (NASM syntax)
https://nasm.us/ NDISASM (NASM syntax)
https://agner.org/optimize OBJCONV (can also brew NASN syntax)

FAQ

https://board.flatassembler.net/topic.php?t=24176
Disassemble most anything, even a letter to grandma, AND FASM assembles the out file.
it does 286 16 bit code and a little 386+ code.
DOS DIS.com is mostly for .com files, but it will do .exe files.
The 100h offset for .COM labels is wrong, and you should load your .EXE into a debugger like INSIGHT and find out where the START is, then you can,
alter without changing size, and FASM assemble it. It will probably run with the new key press values or color changes, etc.
Big files takes a lot of work to make a REAL .ASM FILE out of it, but under 1000 byte executables are kinda easy.