I live in Felixstowe and frequently plan my routes avoiding the A14 (boring and very motorway like).
I would head more cross country stick on the A47 (don’t know that road) to Wisbeach, then the A1101 to Downham Market, the A1122 to Stradsett, then down the A134 to Thetford, then the A1088 through Euston, Ixworth and Norton. You can then either join the A14 to Felixstowe (relatively short hop) or do back roads (can give more details if you want).