Premiere REALLY needs a lossless export option

ProRes or DNxHD at the correct settings are both essentially lossless and way easier to decode. ProResHQ is the standard lossless container for almost every production i've witnessed. The only thing above it is image-sequence based stuff like DPX or TIFF, and honestly id rather have a ProRes4444 file instead.

Try Prores (visual lossless)

I knlw the OP is a year old on this topic but there's an interesting discussion about this on reddit that might help at https://www.reddit.com/r/premiere/comments/9w8aec/help_best_settings_to_export_a_lossless_videofile/

Using long-GOP codec/format for "lossless" encoding is a bit of an intriguing request. Typically, one goes to a mid/upper ProRes, Cineform, or DNx variant to get actual lossless.

Have you tried setting the options for the H.264 encoding? As you can choose all sorts of things including Profile and Level and even to set it to all-intraframe encoding.

I've seen demonstrations where someone set the H.264 settings carefully, and then exported a file like four times, reimporting and exporting ... then applied that fourth generation file to V2, set the blend mode to Difference ... and had a couple pixels show total in a five minute clip.

That's pretty lossless.

Neil