I know the XT series is popular (XT30, XT60, XT90 where the number is the amps rating) for lithium battery packs. Presumably they're designed to handle vibrations without failing. Solder-style (BYO heatshrink). There are both in-line and
panel-mount variants.
LCSC stocks a variety but you probably want something more local/faster if your grass is getting thick, so the jaycar tax might be OK.
No personal experience, sorry. If you don't get much luck here then try some UAV/FPV forums, they will have extensive experience in hooking up lipo packs to high current draws (just like your lawnmower). Also might be worth searching those forums for "XT30 failure" or "XT30 melt" to see if there are some common downsides.
EDIT: For solder-on connectors like this I wouldn't necessarily jump to a bigger size. If you don't have a big enough soldering iron then the joint on the bigger connector might be worse than on a smaller connector.