What's a better investment? I'm looking for some advice for family members

Need advice for family member pls help

Hello
My mother owns 50% stake, and my uncle the other 50%, in an 2300 sqft office building that's been in the family since the early 1960s. It's paid off and it's been rented out since the late 90s.

Nothing has been done to it in years. It currently generates her net about $2000 a month.

The property is appraised at roughly 1.8 million as it sits.

Both parties (my uncle and my mom) expressed interest in selling it soon. I figured the capital gains would be insane though. I was floating thr idea by her of parking the money all into VOO or SPY and it should In theory pay about the same as she's making now from net rent with no real upkeep or maintenance costs.

My very rough numbers are if they were to sell it for around 1.8 let's say, that' would leave each of them with around ~500K net when all is said and done , and 500K in VOO or SCHD *should* conservative generate around 2K a month.

Thoughts? Anything I'm missing?