Are there any actual Original Game Soundtracks to these?
No, not as separate releases. Not until DQ7.
OLD prints of the DQ1~6 Symphonic Suites would often include an "Original Sound Story" track. This would be a 10 to 30 minute medley of the music as it sounds in-game. But it would also include sound effects, and basically sound like an abridged run-through of the game. Town music, stairs sound effect, castle music, bleep-bloop text box, battle transition, etcetc...
That's the closest you'll get to an OST for DQ1~6.