If I have time, I'd like to:

• use some kind of ArrayPool buffer(s) for the codeword ECC blocks
• make a custom BitArray that uses ArrayPool internally for two larger allocations which are internally freed
• decide if there should be any changes (or bug fixes) for strings - see [WIP] Fix character encoding when EciMode is specified #514 - which I have not gone back to yet
• review additionally targeting .NET Standard 2.1, as there would be native Span<> and ArrayPool support for older frameworks, I believe

I have nothing against releasing 1.5.2 at any time you wish -- the performance optimizations have no effect on the exposed API or its use, and any future performance enhancements will be minor in comparison.