Part of the reason why Mojang didn't implement Lava-logging into the base game is probably because they didn't want to have to handle how water and lava next to each other reacts.
Luckily, the solution is very simple... You simply need to add 2 additional possible block states: Obsidian-logged and stone-logged.

Obsidian-logged could make the block blast resistant, but make the block require diamond pickaxe to break (also take just as long as obsidian to mine as well). If you decide to implement this by making it so the shapes of the blocks don't change, then this could be a balanced way of creating blast resistant stairs, slabs, etc...

Stone-logged would make the block have the properties of stone. I would recommend this make whatever is stone-logged become a full block. There would be little purpose to stone-logging a block on purpose.

Both obisidian-logged and stone-logged blocks would drop obsidian/stone respectively.

As a mod that adds lavalogging, implementing these lava-water interactions are a must!