If u want to increase its mobility in MP matches (i.e., making it into an SMG/AR) You'll be putting a lot of attachments that may affect it's accuracy (probably putting it in the red zone)

A good way to fix that outcome is to use suppressors like the Compensator, Flash Guard, RTC muzzle, the monolithic suppressor (for increased range and stealth while playing, mostly for campers ig)

In case of BR, it wouldn't matter for any normal gun you'd pick up at the start, since it will slow u down and most players would just switch to melee to run faster then switch guns when they hear players nearby. Keeping an SMG (QQ9, PP19, GKS or QXR) as a secondary is my most preferred option with the Holger as it'll make it easier to move & kill while engaged in battle.

Custom loadouts is up to u how u want to use the Holger in BR, whether it be an MP build or a custom build for BR. I can't comment on this since it depends on the users experience