Hi,
I cannot make any statements about this particular case but most open source drivers of any sort can and will show some weirdness compared to proprietary counterparts. If we add in the mix that we have 2 different firmware's for that particular WiFi 5 chip then the situation complicates. Some are saying that ath10k-firmware-qca9984-ct is better with total number of connections and devices while others claim that non CT is better. Is there a chance that firmware on dd-wrt is different? You will remember i wrote above you can fiddle with firmware because there have been some instances where forum members discussed about firmware and similar Archer devices. Example: Please consider ath10k (non-ct) drivers instead of ct
As for erratic download/upload speed: i noticed this on my mt7621 router and wrt1200ac as well. It starts with one speed and then it fluctuates like crazy at the beginning however as time passes by, it stabilizes. I could not bother myself with searching for a fix because it never really caused issues however i noticed something quite interesting: if you turn on sqm of any sorts (simplest one, least resource intensive one) then the networks is quite stable. If you think about it, it makes perfect sense however i lost about 25% of my total speed at some points with that turned on so i gave up on that device as in demoted it in my small home network hierarchy. That is a bit different on my ath10k wifi (maybe because of AQL? Have no clue tbh but i read whole thread here and it looks like it is the one which helped me)
Also, some experienced and knowledgeable folks here might tell you that QCA9563 is really pushing the limits with 200Mbps+ speeds. I know i am pushing it with 200/80 and i know some ways to produce high sirq (software interrupts) without much effort.
Curious about something: have you followed cpu and resource usage on dd-wrt? Compared to openwrt was usage lower or? On my device with almost exact hardware like yours, it works fine mostly and i rarely hit 100% cpu: most devices are "slurping" the internet at my home, i dont have some crazy streaming/low latency gaming/website hosting combos like others might and everything is wireless at my home. However i do not use WiFi 4 at all. I have tried WPA3 but the cpu was in more lockups so 100% sirq was fairly frequent.