Can I open an HSA at a new brokerage without maintenance fees? Do such firms exist?
Yes. https://20somethingfinance.com/best-hsa-account/ (mirror) has a nice table comparing fees between HSA administrators. A few of them do not have any maintenance fees. Current snapshot:

https://www.hsasearch.com/compare has a longer list (currently lists 553 HSA providers).
To move your money between HSA providers:
From https://medium.com/@livelyme/hsa-rollovers-and-transfers-demystified-a757c9d7fc4e (mirror):
The IRS allows each HSA account holder to “roll over” their funds to a new HSA provider every 12 months and maintain the tax-advantaged status of the HSA.
https://finance.yahoo.com/news/hsa-rollovers-step-step-guide-140838734.html (mirror):
You can make as many trustee-to-trustee transfers as you wish.
https://livelyme.com/rolling-over-your-hsa (mirror):
HSA Trustee-to-Trustee Transfers vs. HSA Rollovers: HSA rollovers are designated as a transfer from a trustee (like a bank, financial institution or HSA provider) to an individual and back to a trustee (like your new bank, financial institution or HSA provider). In contrast, HSA transfers are from trustee to trustee and there is no limit on how often you can do this
Beware of taxes in case you have to liquidate some of your positions as some US states taxes HSA gains: https://medium.com/@livelyme/hsa-rollovers-and-transfers-demystified-a757c9d7fc4e (mirror):
If you have your HSA funds invested in liquid securities (e.g., stocks, bonds, mutual funds, ETFs, etc.), you can inquire about an in-kind transfer. This is when the investment partner where your HSA funds are invested will transfer all your existing positions to the investment partner of your new HSA provider. Be warned that most HSA providers don’t allow for this. For those who don’t allow your funds to be transferred out using an in-kind transfer, unfortunately you will need to liquidate your funds before moving them. This is particularly unfortunate because in certain states (e.g., California), any interest, dividends, realized gain, etc. is considered taxable income and you’ll have to pay state taxes.