I think that your email closings are too informal. There is no reason why a business email should be any less formal than any other business correspondence. It should close with 'Sincerely yours' unless the recipient is a familiar member of the other firm and someone with whom you have some personal familiarity beyond that of pure business correspondence.
If the latter is the case, then 'Best regards' is appropriate. If you have an even friendlier relationship-- to the extent that you know the recipient's family, for instance-- then you can advance to 'Best wishes' or 'Cheers' or 'Later', etc, which otherwise should be limited to personal letters to friends and acquaintances.
Business English is very rigid in its use of fixed phrases, and there is no reason for angst because you feel that you must vary the very standard but very acceptable 'Sincerely yours'.