How do security pros manage their investments?

You've correctly realised that you want to keep the attack surface as small as possible by limiting the services that can connect to your investment accounts.

Given this constraint the easiest way is a spreadsheet that you update manually.

Thinking about it, if your portfolio was fairly simple you could write a script that calls the yahoo finance, zoopla and binance APIs every week or so to see the progress of your investments. You then manually change the script whenever you make an unscheduled trade, or get charged fees. The script knows how many units of each asset you have, but it's not a security risk because it doesn't connect directly to any of your accounts.

Offline spreadsheet tracking cryptocurrency balances and prices. Spreadsheet is encrypted at rest.

I yanked my mint.com password a few years ago both in frustration and in the hope of improving security. I use Wealthfront to track most of my investments - but I'm reluctant to give them credentials to log in to other financial services.

I loosely track my other financial accounts whenever I log in to them. Effectively I assemble the "holistic view" manually, only once a year, when I do my taxes.

GNUCash on Linux on an encrypted hard drive on a computer that doesn't get connected to the Internet. It's a lot of manual labor. (I used to load OpenBSD on a laptop each and every time I performed a transaction on the Internet. And probably had a minute before my computer/router started being attacked.) To keep track of stocks on a minute to minute basis I use a watchlist on finance.google.com and I keep a spreadsheet on Google Sheets. Thereby reducing the times I have to sign onto the brokerage's mobile app.

Fidelity Net Worth tool (tax advantaged and taxable accounts), Fidelity can also connect to Coinbase to show your crypto assets in Fidelity. RE, PE/angel, etc using a Django app (think brittle homegrown version of Carta).

Excel