Don't know if someone tell and if you know already about this ? but here maybe that what you can try in the Fritzbox which you can find in FRITZ!Box Support under Hilfe und Info and scroll down...
Upstream Optimierung
Manuelle Daten eingeben
TACK Count
1
TACK Period in Jiffies
1
Paketbeschleunigung
Paketbeschleunigung
Layer-2-Beschleunigung
Hardware-Beschleunigung
Referenzbeschleunigung
Paketbeschleunigung inaktiv
Which you can try but not much hope cause for me these all settings literally does nothing and dont bring any improvement but maybe for you ...

I already tried everything possible with those Fritzboxes...
But the first Option is exactly what you talk about before here and about Packets bundling:
Upstream-Optimierung" and "TACK Count" relate to network settings, specifically in FRITZ!Box routers, which control how the router sends data packets to reduce latency, particularly for online gaming. The TACK Count determines the maximum number of subsequent acknowledgment (ACK) packets that can be compressed into a single bundle before being sent. Setting the TACK Count to 1, along with the TACK Jiffies to 1, instructs the router to send each individual data packet immediately, rather than waiting to bundle and send multiple packets at once
How does it work?
Default setting:
By default, the FRITZ!Box bundles multiple packets (often four) and sends them as a single unit.
Optimized setting:
By setting the TACK Count and TACK Jiffies to 1, each packet is sent individually and immediately upon receipt.
Why is it used?
Reduced latency:
Sending packets individually minimizes the time they spend in the router, leading to lower latency and better responsiveness.
Improved online gaming:
This setting is particularly beneficial for online gaming, where quick reaction times are crucial.
Pls tell if this makes any change for you and Online Gaming ? and if try change these Options then just Resync Internet (Neu Verbinden) but don't reboot router...cause after reboot these settings like somehow are at default again...Idk why that happens...