Dealing with 150 fields is always going to be tedious. Whether you spread it horizontally over a single line, or vertically over 150 lines.

The nice thing about the prefix/suffix idea, is that it can be embedded in a standard Xsv file and normal Xsv handling can still be used. If the processor uses the header line, the fields just carry some extra information. If it discards ths headers, it just gets discarded. If it uses the field names for processing, you only need preprocess the first line to strip the suffixes to allow it to still work.

Things that a YAML/XML/Other format description would never allow.