you can set strong(delta: ...) to a new value; see:
The default delta is to make the font heavier by 300 (whatever that weight unit used by font is called).
Note that fonts come in discrete weights – or rather, Typst doesn’t yet support variable fonts – so setting e.g. 400 may not change anything since the nearest available weight may still be the same. In that case, increase the change until it works.