Or cartel like behaviour? Leader needs to look tough to survive.

Donbas region is a major industrial area for the past century.

No surprise Russia wants it.

Russia has controlled a substantial portion of the Donbas for 8 years and has simply let it rot. There's no investment or utilization of that industry.

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

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".

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.

The eastern part of Ukraine has vast resource deposits that are worth trillions. What makes you think resource competition isn't a part of it?

The original reason for invasion (2014) was access to the sea. Logistics are crucial for an economy.