It sounds like you now have an ordinary brokerage account. You will pay taxes on any cap gains that are deposited into that account whether from mutual fund distributions or stock sales. Even though the inherited funds weren't taxed at the time you received them, any money you earn from them going forward is fully taxable.
You can only contribute a Roth IRA if you have earned income and it is below $140K ($208K if married filing jointly), and even then the limit is $6K ($7K if over age 50). Since you are currently retired, it sounds like that's not possible. Since you have rollover IRAs, even a backdoor Roth contribution, which gets around the income limits but not the earned income requirement, wouldn't work for you.
One option is to migrate this money into a fund that is designed for growth and avoids throwing off income and just leave it there. You would take the tax hit as you sell off any of your current investments that have grown, but once the money is reinvested you wouldn't owe any tax until you withdraw from the account.