Key isn't working consistently/ no start issue

I recently got a 2017 Ram ProMaster City used. A few weeks after purchasing, I was driving down the road and when I had to brake, the engine cut out. My Dad came to assist (he was an army mechanic back in the 70s and worked on all of his and my vehicles through the years.)

Turns out there was no oil left! He added 2.5quarts of the appropriate oil. He also discovered that the battery terminals are corroded and there was a nail shoved in one of the terminals to complete the connection. He removed the nail and tightened the connector.

The van took a few minutes to respond to the oil, and then it turned on with no issues.

However, since that day: if the van has been sitting overnight, or after a long shift at work: when I put the key into the ignition and turn, there is no restroom the engine. I hear a clock, the dashboard lights come on, but there's no startup. I turn the key a bunch of times until it finally responds and the van starts up. Sometimes it takes only 2 attempts. Sometimes it takes 8, or anywhere in between.

My dad wonders if it's an issue with the battery, or an issue with the solenoid starter. He's been looking for his battery terminal brush so he can try cleaning the corrosion off the terminals.

He's not super familiar with such a new vehicle and he wanted me to inquire from you all in this forum!

Thank you so much for any thoughts! Has anyone else ever had this issue?