If your local datum is something like mine (ETRF89=EUREF89) and your transforming parameter for WGS84 is similar.
The WGS84 will state a transforming accuracy for geografic coordinates to be +/-1m. Wich is less accurate then actual local point that could be 0-100cm away. Transforming geografic coordinates is not 100% accurate.
If you compare the defining parameters for WGS84 and ETRF89, they will most likely state 0 or equal in Qgis.
Check out WGS84 and difference between geocentric, geodetic and geografic WGS84 datum
I showed a step by step transition from WGS/ITRF to EUREF89 (based on a master thesis somebody els wrote) taking the earth`s change over time into consideration. This is just a step by step .
I can run all this in one batch with a dedicated local transformtion tool (SKtrans) with just two click but the point was to show there is more to transition then just convert it from XXX to XXX.
As you can see, it doesnt cut it by just use WGS84 to ETRF89. The defining parameter isnt good enough. Qgis could probably do it but i havnt found a way yet.
I guess that you need a tool that does it for you. Problably the top brands out there does it.