Among the strangest quirks of Indian history-
After Alexander's battle with Porus, the next major battle where we have minute details of how it played out are the Ghaznavid battles with Shahis of Kabul in 999CE - that too from Ghaznavid historians.
A gap of 1300 years!

The Maurya conquests, Gupta-Hun wars, Satavahana-Scythian wars, Harsha-Pulakeshin, Pratihara-Rashtrakuta-Pala wars are all known only from inscriptions of few lines.
Details of battles were always valuable experience for princes and generals. How were they managing w.o it?

they could be hiding in plain sight. Some campaigns of Gajapatis are described stylized as Mahabharata. Much earlier, Raghuvamsa is similar to the Gupta campaigns.
A number of such details may be in poems or stylized, apparent to the people when composed, but forgotten now.

"it was all destroyed" is hard to believe. especially when other textual material does survive. this would be of great importance even to a foreign invader.
as a counterexample, almost all of the Sassanian texts are lost after the Arab conquests, so are Carthage's texts.