As for addresses, I could in principle use my brothers address in Seattle, WA, but I've always been wary that I'd then be chased for state taxes, and from your experience it sounds like my concern has been justified.
WA is one of 7 states with no state income tax, relying instead mainly on property and sales taxes. However, starting 2022, they are introducing a capital gains tax of 7% on gains exceeding $250,000. Unlike most taxes, that $250,000 allowance is NOT increased for married couples etc. On the plus side, tangible property outside the state is not covered by this tax, your WA home (if you have one) is also excluded, and WA is not one of the 4 sticky states in terms of severing ties with it.
WA has been trying to institute a capital gains tax for years, but it keeps being voted down by the populace (probably too many Microsoft, Boeing, Google, Amazon, and Nintendo families in the area). This time around, they found a 'emergency need' clause that let's them avoid having to let their constituents vote on it. The new tax is being challenged in the courts and likely to come down to whether the courts decide this is a tax on property, or a tax on exercising a right related to a property. If it's the former, it's incompatible with the state constitution and will be struck down (and surely modified and resubmitted next year). If it's the later, it's completely legal.
Source: I live in WA :-)
So unless you have a lot of capital gains, I think your brother's address is a good one to have.
If you still have concerns and a need for a US address, you might want to look at Virtual Mailing Address services.
I'm leaving the US for the UK end of Feb 2022, and I've gone ahead and signed up with physicaladdress.com* for one of these. This gives me a physical street address in Florida (not a PO Box) at which mail and packages can be received. When mail arrives, I receive a scan of the envelope and the option to have it opened & scanned, forwarded, thrown away, or shredded.
I selected Florida as it has no state income tax, no state capital gains, and a low sales tax (nice for paying fees for online subscriptions services I don't intend to cancel). Texas is also very popular, but I can't say why I picked FL over TX without straying into politics and my opinions on what's happing to women's healthcare in TX at the moment.
Places that won't take a UK address will get my FL mailing address. In case I failed to update someone, I'll use the free USPS change of address forwarding service to direct mail sent to my old WA address to my FL mailing address for 6 months.
Once I surrender my WA state ID and terminate my lease, I'll have nothing tying me to WA state anymore.
* PhysicalAddress.com is just one of many similar services. I haven't actually received any mail there yet (I keep meaning to send a test letter!), so I don't know how good they really are yet. I can say I did a fair amount of research before selecting them, and that sign-up was a breeze (even the bit where you get a on-line notary for the USPS form that lets them open your USPS mail). Google 'virtual mailing address service' and you'll find a bunch of companies offering similar services for similar prices.