@Prodeje79 - I presume you've ruled out other pitfalls, such as poor-quality layer 1, saturated LAN links, wifi interference/repeating, etc., and also confirmed you're indeed saturating your download and/or upload when these behaviors are occurring? If reasonably sure it's a "yes" to both, then properly-applied and high-enough throughput SQM on the WAN interface may indeed solve your issues.
AFAIK, the problem with trying to do any form of SQM on Broadcom hardware (most Asus models, the 68U included) is kernel driver integration is not 100%, due to Broadcom's closed-source drivers. I'll ping
@RMerlin and
@ttgapers, who just came out with a cake scripting extension for Merlin -- both of which could probably confirm, or correct me if I'm mistaken. If largely correct, though, that discrepancy could potentially help explain why one might notice sub-optimal de-bloating on Merlin, as opposed to with OpenWRT on Qualcomm embedded or x86_64 architectures.
Regardless, 430Mb/s SQM would be doable with higher-clock MIPS and ARM routers (EdgeRouter 4, UniFi Dream Machine,
maybe an R7800, etc.), but for 1Gb/s+, you'll probably want an x86_64 box, 2Ghz+ clock Celeron or i-Core CPU.