Anyway to do this and not lose all the blocks? I mean, I can live without them. But if i missed some quartz Id be upset.
Yes and no. Yes if you leave it alone a filler WILL destory all the blocks without exeptions. However if you stand right ontop of the blocks get get broken you can scoop them into your inventory. However this requires you to watch really well whenever it is approching quartz and quickly move closer to be able to get the quatz. But in all honesty you probly wont be able to get it.
If you want the blocks that will be mined away you have use eiter a quarry or a turtle. With a good turtle program you can tell it how deep to dig so it will stop where you want to. But without codes you could use a quarry, simply make a lava lake under the digging area at the level you want the quarry to stop. When its done remove the quarry, dump water over the lava and replace it with your floor of choice.