It also reduces supply of many resources (for example noble gases since Ukraine is the world's biggest exporter and Russia is the second), leading to higher prices which Russia would profit from (neon prices went up 600% around 2014). In the current situation, they also didn't expect to have to wage a year or longer war or deal with crippling Western sanctions.

It's only worth investing once you've really secured it against the other side retaking it.

I don't think the Donbass was ever secure against being retaken. Hence no investment.

It's still strategically valuable, because you're still taking resources from the enemy.

It also wasn't officially Russian territory. They were still playing the game of "these are separatists that just want to join Russia", so it wouldn't make sense to invest anything in what's really been an active warzone for 8 years. Officially annexing Donbas was probably always the long-term plan from the beginning, but for some reason that turned into "fuck it, let's take Ukraine instead".

“The Donbas represents one of the largest coal reserves in Ukraine, with estimated reserves of 60 billion tonnes of coal.”