Besides heat, WD-40, and more leverage, the standard thing for removing rounded-out stuck bike bolts is to gently hammer in a slightly-too-large TORX bit which will bite (despite the damage), and unscrew.
You might damage the bit, but it's only a few euros spent - for a perfectly reliable technique.
talldaveos t1_j6heh5d wrote
Reply to How do I remove a stuck bolt holding my bike's kickstand in place? by S_Orbital
Besides heat, WD-40, and more leverage, the standard thing for removing rounded-out stuck bike bolts is to gently hammer in a slightly-too-large TORX bit which will bite (despite the damage), and unscrew.
You might damage the bit, but it's only a few euros spent - for a perfectly reliable technique.
Rather than here, you could try https://www.reddit.com/r/bikewrench/