If you want to limit the quality level found in loot, you would need to change the probability tables QLTemplateTO, T1, T2, and T3. For example, in T1 table:
<qualitytemplate level="99,100" default_quality="1">
<loot quality="2" prob="0.264"/>
<loot quality="3" prob="0.315"/>
<loot quality="4" prob="0.397"/>
<loot quality="5" prob="0.552"/>
<loot quality="6" prob="1"/>
</qualitytemplate>
I made it harder to find the higher quality equipment in loot so I ended up changing the probabilities at one time.
<set xpath="/lootcontainers/lootqualitytemplates/lootqualitytemplate/qualitytemplate/loot[@quality='3']/@prob">0.45</set>
<set xpath="/lootcontainers/lootqualitytemplates/lootqualitytemplate/qualitytemplate/loot[@quality='4']/@prob">0.35</set>
<set xpath="/lootcontainers/lootqualitytemplates/lootqualitytemplate/qualitytemplate/loot[@quality='5']/@prob">0.25</set>
<set xpath="/lootcontainers/lootqualitytemplates/lootqualitytemplate/qualitytemplate/loot[@quality='6']/@prob">0.15</set>
Another route you could go is remove them completely and then insert your own custom tables. Probably not be very difficult since you would only have a couple of quality levels so you can expand the loot stage range so less sub-nodes you would need.
I went a different route (I had the same issue - once you find that Q6 weapon, why bother with crafting or searching anymore? I went a different route in my playstyle where I removed the ability to repair equipment. So you ended up having to buy new gear from traders or craft your own (I am right now testing out my modlet that introduced Q6 crafting), or using that halfway damaged weapon you found in loot until you can get something else to replace it.
Also if you want to, you can look at any modlets I have created for people over A20/A21. You can just search for BFT2020 at github.com and it will take you to where I posted them for download. There are currently 24 in there.