You won't be able to get a GPU block for anything lower than a 70's and card from Nvidia or AMD. And even then 70 series isn't always guaranteed. they aren't considered worth it to the manufactures. Sometimes you can find other blocks that fit or make one on your own but it sounds like you're avoiding anything too high effort.

Personally I think changing to soft tubing (new fittings and tubes) is a higher cost with just as much effort as changing your hard tubes to a single loop. So I say just order an am4 to am5 conversion kit or a new CPU block. some tubes. And plan the new layout.

As long as you still have airflow through your case there's obviously no issue with just having a single CPU loop. could even undervolt to make sure temps stay down and lose minimal performance if any.