private as possible

What are you trying to hide, and who are you hiding it from?

I dislike that some privacy forums, like reddit and therefore here by proxy, have a cultural habit of talking about privacy or security as an abstract value in itself. But when we start getting into more detailed questions, it's all vague and vibes until we make it clear who we're trying to hide from and what we're hiding.

For example, most of the time I'm not hiding from my own government. Sure, I incidentally do make it a bit harder for them to track me, but I'm more focused on hiding from Meta/Alphabet/Amazon/etc. (plus from a small group of deranged online stalkers obsessed with some of my friends) so there are plenty of online services and stores I can buy from without taking inconvenient measures. It's fine for me if some services can guess my name and know where I live and one of my phone numbers. It's not fine if they learn some other details.

It's important to get out of the habit of saying "more private", "less private", "most secure", and talk about what you're specifically concerned with and how tactics and tools specifically address that. What information will Google gain from knowing your investments? Is that a threat to you? Are there acceptable ways to mitigate that threat?

Using google to create an account would mean giving them my real data

I've been surprised how easy it was for me to make a fake Google account with no links to my real identity. I only use it for age-restricted YouTube videos, I wouldn't trust it with money like investments, because the way I set it up is inherently suspicious and I wouldn't be able to verify identity if challenged.

No.

Not because it's not technically feasible but rather I would psychologically not manage to make money knowing my portfolio, either directly or via EFTs, makes me money by profiteering of BigTech or surveillance capitalism.

Full disclosure : I did have Apple and NVIDIA stocks and I did sell them not because they were not making money (there sure were) but because I felt disgusted by HOW they made money.

PS: KYC and related laws in a lot of countries demand you use your real information and declare your earnings, so again it's not a technical problem, it's at least ALSO a legal problem, and arguably a moral one if you believe KYC kind of laws help to curb money laundering.

invest engine or vanguard:

you can just use the website, dont need the app!

I keep separate devices and networks. It is not privacy as much as it is purging suggestive content and convenience. I only purchase something when I go looking for it specifically. I never browse or shop as entertainment or for endorphins. Investment apps are all about the stalkerware because it says you are a major target for unnecessary nonsense. Get a tablet and only use it for that junk.

The fact that you're still using Android 8 makes everything else moot, seeing as it hasn't received a security update in nearly 5 years. I would take care of that before anything else.

Any legitimate broker is definitely going to have a desktop site, though.

If you are in the US, take a look at Fidelity or Vanguard. They haven't required the use of a smartphone app.

Using a phone with Android 8 isn't best practice for security by any means, but unless you are being targeted or going around downloading shady apps, it's more likely it will run into app incompatibility issues in the coming years than anything else.

For sites where I'm making a low-value, one-off purchase and never coming back, I'll use a pseudonym alongside a prepaid gift card, or failing that, a privacy.com virtual card. Not quite a sustainable strategy with eBay or Amazon, especially if the package needs a signature, so I'll just use a privacy.com virtual card and supply a P.O. Box address

Mostly accepted that it is the way it is for these things. If the privacy-friendly option is giving up a few conveniences, I'll take it. But if it's keeping me from reaching certain goals, I'll tolerate a compromise. I don't think I'm being targeted either, so it's all tolerable in my personal threat model.

I would consider Fidelity or Interactive Brokers.

Buy monero. No need for all that pii