I agree that you should buy a few different boxes to try out. Also, think about what your average shot distance, target size, target geometry, etc. when you start checking out ammo.
In my experience, factory Hornady used to be the easy button, and I've previously bought 1,000+ rounds of factory Hornady ammo even before receiving a barrel because I knew it would just work. But post covid, my experience with Hornady white box factory ammo is that precision is sub-par and SDs are pretty big (20-30 fps) for 6 and 6.5 CM. Hornady Black and Hunter has been better for me more recently. Federal Gold loaded Bergers and SMK tend to be more precise and have about half as large of SDs (10-15 fps). Berger loaded ammo has been a mixed bag for me. Sierra competition has been good in 6CM but VERY slow in 6.5CM. Norma hasn't been good in any of my 6 or 6.5 CM barrels, but it's cheap. Nosler, Black Hills, Sig, and HSM has been bad too. Lapua loaded 136 scenars have been good out of multiple barrels.
If I had a new 6.5CM barrel today and wanted to use factory ammo, I'd buy whatever ammo to get 100- 200 rounds down the barrel, then I'd try out:
Federal 130 Berger OTM
Federal 140 Berger Hybrid
Federal 140 SMK
Hornady 143 ELDX
Berger 140 Hybrid
Note: I'd only buy the above ammo from a seller that had at least 800 rounds of same lot, so that I could turn around and bulk order 800 - 1,500 rounds once I determined what I wanted to use.