Intel microcode updates for 1803?

Not until 1803 is released to the public.

This is actually a bit of a misnomer. If your PC (or motherboard) has received a recent (early March or later) firmware update from Intel, AMD, or the manufacturer, you're probably OK. If you've been depending on various security updates from Microsoft for that coverage, you may find yourself uncovered after upgrading to 1803. The best way to find out is to run Steve Gibson's InSpectre utility, and see what shows up as a result:

I just ran this on my production PC which received a firmware update from Asrock in mid-March. As you can see, even though this system is running 1803, it remains protected.
Keep calm and carry on ... with the patches!
HTH,
--Ed--

MSI may issue another patch or Microsoft itself may do so. All you can do is keep your eyes peeled on Windows Update for something from Microsoft, and the MSI website for something for your specific motherboard.
--Ed--

It'll be awhile before ANY new microcode updates, since even more(8to be exact) Spectre flaws were found: Exclusive: Spectre-NG - Multiple new Intel CPU flaws revealed, several serious | c't Magazin

So don't hold your breath, they'll be out, when they're out.
Or is it better that Intel and the OEMs rush it, and cause even more system stability problems, like the first updates did?

in the past year intel has released bios security fix in the following order: imei -> spectre -> sfi (edit... whoops.. that was meant to be SPI)

when your mb maker will release bios that include those fixes is anyone's guess. even after they release a new bios, you probably won't even know whether they've included the fix... (eg.. asus has terrible patch notes/change logs. it released a bios at around the same time as the spi fix was released by intel. no clue if it's actually included!)