It's hard to say how much speedup I really got, because mobile network reception is very unreliable here. But Wireshark confirmed both connections were being used and it seemed like a ~50 Mbit/s 5G NSA connection was combined with 50 Mbit/s VDSL to make 95 Mbit/s (I ran two speedtests in quick succession to compare them). I feel a bit silly for how much I researched different ways of combining multiple connections when all it needed was a simple TCP proxy.