@Zoopet are you worried only about being lopsided after mastectomy or also after lumpectomy? I can't advise on mastectomy (others on here can though) but I can tell you about my WLE (lumpectomy) and the repair job.
Do you have a breast cancer nurse team? I was able to book an appointment with a BC nurse to look at outcomes of different treatment options (i was having WLE but the tumour was in an odd place so tissue loss was an issue). I could have just had the lump taken and no reconstruction, lump out and some reconstruction using belly fat, lump out and a lift to even out the hole, or lump out with a reduction and a symmetry reduction (therapeutic mammoplasty) on the other breast.
I had massive breasts (hated them) 38 H so went for the last option. It has worked out well for me (i have yet to start on radiotherapy though). At my hospital they actively offered all these options and wanted to do the symmetry surgery in the same operation, so no waiting around lopsided. Its possible not all hospitals will offer the same options.
Wishing you the best! How are you feeling after your treatment so far?