So, after line battles were beginning to become obsolete and before World War I became trench warfare, what was the common infantry tactics? Was it still a variation of line battles?
Kind of, for a given definition of 'line'. More can always be said on the matter, so if anyone passing by has more to say, go right ahead! For the meantime, we do have some previous posts on this matter:
On the matter of how WW1-era battles were fought before the digging-in period, we can look to the appropriate section of the FAQ (1914, if that doesn't automatically take you there); specifically, this particular post also by u/elos_, on this matter precisely.