Most important

All IEs as value types

use offsets instead of pointers to reduce memory footprint and allow simple copy

Review code to reduce code-bloat

Remove excessive template parameters
Common part of buffer via type-erasure + trampolines
Non-templated allocator
Introduce .cpp files?

Medium importance

More concepts to improve errors

Minor importance

Independent decode/encode in UTs

it's much simpler to debug encode independent of decode

accumulating print into span