This may be personal opinion, a bit, but I would not recommend modifying your data's coordinates until the production database that you store the data in can handle the new coordinate reference system.
- Discoverability: if you store data in there now, you can't attribute the data with the correct CRS, you'll be saying it's GDA94 still, which is wrong, misleading and dangerous. Sure, the coordinates are 'better' if they're GDA2020, but they're not going to be right if you label them with GDA94.
- Deadlines: sure, people are starting to deliver data in GDA2020, and that's great, and software like FME and QGIS support the new CRSs based on the new datum, but still, there's going to be lots of data around in the old CRS for a long time. So there's no looming deadline.
- Do it once, do it right: working with complex systems is ... complex, and you don't want to have to redo it in your new SQL Server 2020, or whenever Microsoft supports the new CRSs. So just wait. Also, you can use this time to complain to a vendor about it and to encourage them to fix the root problem!
So, what alternatives do you have? Either wait for a version of SQL Server that supports GDA2020, or perhaps explore using something like PostGIS, which already does!