Trying to help 65 y/o parents handle windfall

Hi All, I started out doing some research on the windfalls as well as looking at bogleheads threads with a similar theme but wanted to see if I could get some additional input. My parents are in their mid to late 60's and have a windfall of about 2.3 million after paying off their debts (mortgage, credit cards, etc.) and taking care of mandatory repairs on their home. This is after taxes. My mom manages the finances and is a bit overwhelmed by how exactly to handle the money and invest it to continue to grow without assuming too much risk. My father receives pension and disability and will continue to receive this so long as he lives. He also has his medicine and doctor's visits covered if they relate to his handicap. Between the disability, pension and social security, they bring in around $9k a month (before taxes). They live very modestly (drive cars til they die, don't buy flashy stuff, aren't taking luxury vacations, etc.) and will not all of the sudden be purchasing these things. My brother bought my dad new "house shoes" last year after we discovered his pair at the time had been his house shoes for 20 years.

They want to feel safe with how the money is invested but they want it to "work for them" during retirement. My dad is very much old school and would hold Scrooge McDuck style in a giant vault if he could. I've already passed them any and all advice I can find about financial planners, wealth managers, etc. but would greatly appreciate any thoughts on conservative investment strategies.

Summary:

• Parents are receiving 2.3 million after taxes and paying off debt
• Parents don't really know much about finances and very much like to read things through
• Siblings and I have been investing in retirement but the 30 year gap and amount of money is overwhelming
• Dad is ultra conservative and will want to invest as safely as possible
• Have definitely read lots of windfall articles but I want to help provide as much info/insight as possible.
• If they go to a private banker/wealth manager, they need to be equipped to push back/ask questions
• My siblings and I are all financially stable and we do not have any leaches in our immediate circle. It will stay that way