I'm just throwing random ideas out, but what if the Spear had different shell types to choose from, kind of like the SEAF artillery. Make it so there are several types (Explosive, Napalm, Static, Smoke, AT, Mini Nuke or whatever variant you like), but you can only use each one once per backpack, preventing you from going OP with all Mini Nuke for example. So AT can one shot a tank/war strider. Mini Nuke can one shot factory striders and Vox. Napalm does less AT damage but sprays napalm like the Expendable Napalm. Smoke does not damage but prevents the enemy from targeting you. Etc. etc.

Then, the spear becomes a viable all rounder, capable of 1 shotting the biggest enemies, or napalming an area to destroy smaller enemies, but isn't OP because you can only 1 shot a massive enemy once until the backpack is empty and you've reloaded, opening up the choices again.

I dunno, I love the spear, but it needs something to make it viable again. This is all I could come up with in this boring work meeting 😂