I am retired. File jointly with spouse.  I have a few Traditional IRAs.  Spouse has Solo 401
(1) Do we need to file separate tax returns to proceed with the conversion to Roth?
(2) Other considerations before converting to ROTH?  
(3) Also is there a consideration regarding the year of the conversation?
(4) Please explain what Fidelity (https://institutional.fidelity.com/advisors/investment-solutions/fidelity-advisor-ira/roth-ira/considerations-before-converting-to-a-roth-ira)  means that there is no longer a limit to Modified Adjusted Gross Income for Roth IRA?
(5)  On the 1040 Return, what value (field) is used to determine the tax bracket (https://www.bairdwealth.com/siteassets/pdfs/tax-information/2024-tax-facts.pdf)?
(6) Method to determine amount to convert IRA, Solo 401K to ROTH to continue to (i) be in existing tax bracket and (iii) to not increase Medicare?
Thank you in advance!!