Chunk a large set of characters by a specified set of delimiters and a maximum chunk size
Don't you need to test for delimiters here?  From reading your code I assume we want to max chunk but if inside the chunk there are delimiters we want to break that up.  If the last section fits in the entire chunk it's not checking for delimiters
if (pointer == sourceSpan.Length)
{
    items.Add(sourceSpan.Slice(lazyPointer, pointer - lazyPointer).TrimStart().ToString());
    break;
}

Also you could use IndexOfAny instead of the loop at the end
WARNING this code not tested and just used an example
var slice = sourceSpan.Slice(lazyPointer, pointer - lazyPointer);
var contains = slice.IndexOfAny(delimeters);
while (contains &gt; -1)
{
    items.Add(slice.Slice(0, contains + 1).ToString());
    slice = slice.Slice(contains + 1);
    contains = slice.IndexOfAny(delimeters);
}

Now some other points that I think you should split by delimiters first then chunks.  Instead of chunks then delimiters.  If I have a string of  &quot;12345.6789.8.654321&quot; and split by 7.   The first two coming out would be &quot;12345&quot; &lt;-- looks good but then the next chunk would be &quot;6&quot; then chunk after that would be &quot;789&quot;,&quot;8&quot;,&quot;6&quot;,&quot;54321&quot;  if we split by delimiters first then chunked them it would output what I think most people would expect.