I test using openMSX cfg'd/linked with build artifacts usually, otherwise I burn the ROM to my MFR SCC+ SD using SofaRun. I test using a stock Sony HB-F1XDmk2 with no expansions plugged in (except the MFR).
The interesting part:
The ROM is setup with a standard byte header. It occupies page 1 and 2. Code exists in page 2; page 1 is usually hidden behind Main-ROM, and is accessed when loading data. 4000h-7FFFh will only contained RLE'd game data - graphics, room data, music, etc.
I made a quick run-length encoding script in Python that compresses the game data. The decompression routine is extremely fast making it well worth it (and possibly can be optimized more).
The music player is a variant of the DOS-based MuSICA replayer that I made a few months ago for an MSX2 project that uses OPLL. I cut out what I could from my disassembly that was obviously not for PSG-use and shaved a kB or so. I shoved it as high as I could in RAM and this bad boy fits neatly in E000h, still alotting a ton of space for game-use RAM. I composed the music on my F1XD using the bug-fixed and SCC+-compatible MuSICA editor I released. Music is 2-channel, allotting for software SFX on a third channel.
MuSICA is a little on the slow side though, so it might need tweaking...
The graphics are all designed by me using my MSX2 graphic toolset. I export as .z80 when I'm ready and my build script does the rest. There are a few bugs with the toolset I need to tweak as of writing, like exporting from the screen tool only works with .z80 for now.
The enemies are background tiles a'la Gauntlet I on NES (9918 VDP has too low of a raster limit to make them real sprites). This actually lets them be very colorful on the MSX1 - I used my tool in sprite mode in one window, then copied them to the tool in pattern mode so I could do 16x16 at once. I looked at Final Fantasy Legend/SaGa sprites as inspiration.
That's all for now!
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