I don't work or LH but I have access to a couple of sites with load information. So take it with a grain of salt.

On days you mention the only flights I found are operated by OU that LH codeshare on. On 4-16 the load look good, 4-17 bad and 4-18 good. In order of best availability 4-18, 4-16, 4-17.

Each of your return dates looks good. Again in order of best availability, 4-26, 4-27, 4-25.

Good luck! Remember that this is 3rd party info and not directly from OU or LH.