It's been a busy week for Windows Insiders on the Slow ring. Build 18351 was released to the ring on Monday, and then there were cumulative updates for that build on Wednesday and Thursday. Now, Microsoft has released Windows 10 Insider Preview build 18356 to the Slow ring.
The build doesn't really contain any new features coming from build 18351. There was a note about an update to the Your Phone app, which will now allow you to mirror your Android phone and interact with it. Unfortunately, you need a phone from the Galaxy S8 or S9 families, and you need a PC with BTLE. Touchscreen input doesn't work either, so you need to use a mouse and keyboard.
There's still a known issue in the build where games that use anti-cheat software will cause a green screen crash, so you should be aware of that. That issue requires a third-party fix, so it's unclear when that's coming.
We should see an RTM build for Windows 10 version 1903 before the end of the month, followed by a public release in April.