Sounds like you have no other option for now.
Just out of curiosity, what's a LAGG (with two Gs)? 
"Link AGgregation Group"?
Yes, unfortunately. ZyXEL GS1900-* use Realtek RTL838* too.
Link AGGregation is called LAGG in OPNsense, probably to make clear it's not about delay (lag). https://docs.opnsense.org/manual/other-interfaces.html#lagg
OMG. The entire industry calls it since 30 years an LAG.
Hi there, I am wondering, one year later, if anybody managed to implement the hardware offloading for LAGs for the realtek target / switch driver... Any (good) news?
Apart from the passing of time, nothing has changed (the realtek targets needs a lot of love, but received rather little).
Thanks for the (very quick) answer. I reckon that at this "pace" there's no hope 
Is it implemented for any other switch chipset though?
Sure, for all managed switches except realtek - except that 'all' others is defined as empty set.
In upstream kernel yes (I think? Need to read more code....). In Openwrt I think not.
One would need to have a look at the following DSA drivers i think?
Other than that It was also asked again in the realtek support thread by me and I had a look at the code nothing no. IMO be happy with spanning tree haha. If a developer is willing to do the work there is some code there and one of the developers said they had done some work in one of their git branches.