I mean, like i see a lot of people like rean. He just has a vocal minority who aren't a fan.
Also oh right now that I'm at my computer, it's easy for me to find and link the specific feats that are easily scaleable:
Shizuna's moon slice
I did a rought calc awhile back and got like 900 zettatons (5-B nearly 5-A) but I'm not a calc expert so It could be higher or lower. It at a minimum is 29.6 exatons for obvious reasons. (IE the slash obviously overcame the moons GBE since the two parts don't come back together) And since the slice hit it in less then a second the actual slice is bare minimum FTL (I think I got like 16 times FTL.)
The blog for MFTL+ and High 4-C
(Values of 1318 for speed, and 3.37 FOE for AP)
Hopefully this works but also
Elaine's FTL craft statement
Specific arts that state they are light that are relevent:
Zeruel canon and Cross Crusade
Luminous ray, Atelier canon and again Cross Crusade
Saint arrow and Ark of El Dorado
Emma's S-craft is calculated at anywhere from Multi-continental to Moon level
(82 petatons to 77 Exatons)
Ishmelga's S-craft is calculated at brown dwarf
(271 ronnatons of tnt)
Sky the 3rds final boss blows up the planet in their S-craft
there's also the whole creating a pocket dimension that's at least 4-A due to stars and a moon Possibly a universe from Sky 3rd too. (So 16.88 ExaFoe to Universal)
We can divy up who scales to what later on. But all of these pretty unambiguously scale to Rean.
Be it from being the in the games they are directly from (Lost genesis, Ishmelga, the several light arts, Emma) or Scaling to the character's (Elian shouldn't be faster then character's like Sara (especially when she questions her promotion to A-rank at first), Shizuna pretty much is only interested in Fighting Rean of the 8L1B masters and a few horizen spoilers I won't say. And almost all the Sky cast appears in CS3,4 and Reverie to help and are pretty much not so obviously above everyone else.
There are more feats and statements but honestly I just grabbed the biggest and most notable one's that could easily be scaled.
Pre time skip rean can just be scaled to Emma or 4-A (Because thanks Cold steel 2 epilogue, Lloyd and Rean tie). Post time skip High 4-C or again 4-A.
And Speed wise anywhere from FTL to MFTL+ is pretty consistant.
Even pre-timeskip rean should get it due to the above light based arts.