SaaS managing other's people money: when I need a license, and when I don't. Help

I need help, maybe you can point me in the right direction since this is probably a complex multi-faceted legal issue that varies country-to-country.

Say I want to make an app/platform that needs to pool money from a small group of people and invest them in the stock market on their behalf, i.e. ETFs. I want to provide extra value compared to what they could do by themselves on, say Interactive Broker. For instance, I could make the app easier/better to some very specific/nichey use cases.

Hypothetical Scenario: person1, p2, and p3, come to my website and pool funds with a common goal: say, buy a home 10 years from now. Imagine a crowdfunding campaign titled "Pool money to buy a house to p4 in 10 years" .
I collect money via stripe/PayPal/WireTransfer and use them to buy a Vanguard ETF on their behalf. 10 years later I sell the ETF and return them the principal + gain (or minus losses).

Is it possible/legal? Even with a creative solution, keep in mind I am not a bank but a solo bootstrapper.

For instance: Gofundme is not a licensed bank afaik (read their T&C), they don't hold the people's money for more than a month. Paypal/Stripe does, and then they release it to a bank account when the campaign is over. So I guess the "crowd pooling of money temporarily" is legal.

For the investing part, it gets trickier and we must distinguish the cases, since I'm not a Broker, so I need to either

a. Get a corporate account on a broker, trade using my corp account, then withdraw to my corp bank account, then send them to p4. Can I do this or something similar without being an RIA?

b. Get one of the pool users to create an account for themselves on an existing Broker and provide me their credentials/API keys, so I can automate stuff, create reports, functionalities, and act on their behalf.

For the latter case, I've seen and used apps asking my IB credentials /API keys, and doing stuff for me (think trading bots or portfolio watchers) ... none of these seemed to have banking licenses or other licenses... Just disclaimers and T&C.
I've also seen financial apps that connect to my bank account to give me a credit score and aggregate data from multiple bank accounts, and they DO have big-ass licenses in their footer ;)

I'd like to know if some of you fintech hackers can point me in the right direction. What would you do? How far can I go with a solid T&C and disclaimers, vs what I can't do without a proper license?

What fintech services do you suggest I use to abstract from this complexity and be able to say "look I don't hold your money, I don't make investments for you, I use THIS and THAT licenses services with insurances that do it all right for you. I'm just an app that automates very simple operations without taking investment decisions without your consent"

And in the latter case, I am not giving up. Is it feasible, or only available to Hedge Fund Managers/banks with AUM > 35 million dollars?

Thanks