Lubeck...
I can understand your frustration...Especially in light of involvement vs.return...
and I'm sorry things aren't working out as planned, but can assure you these are not insurmountable obstacles..
The frustrating part is we/you are fighting battles on several fronts at the same time which may or may not be connected..so bouncing back and forth takes up a lot of energy...
Dude..you need a vacation...
I'm sure we are overlooking something rather minor, probably obvious, that is hard to see when focus is drawn in several areas at once...So tackling one problem at a time is probably a better approach...
I can tell you what it's NOT... which will help narrow down your/our focus to what it may be..
It's not the bubbles...as stated earlier, though annoying, not harming DT..
It's not the sump/fuge tank..That particular design, layout, even measurements are working in Leonas system just fine...
It's not the "additional water volume" causing cycle...(unless there's a problem with source water)...otherwise it's just a WC dilution of elements...
So that leaves us with:
1. DSB issues..Crabs digging, Overall health and age of bed and balance of micro flora and fauna "life" keeping bed functional...
2. Uncured LR possibly causing mini cycle and resultant reaction of corals..
3. Source water...silicates Diatom bloom etc..
4. Drain system function, drain capacity and source of bubbles in sump...
The 4th one I can help you with...
I believe the issue is the long vertical drops you have (per photos and earlier "theory") on suction effect and possibly the "H" pipe (crossover manifold) I mentioned earlier several times and am not sure if you installed or how you have that laid out...
So here is the (hopefully) solution to vertical drain and cross over/bubble issue..
Cut the two drain lines just below floor(ceiling) level ...45* toward center..."T" into larger crossover "H" manifold... valve each end... drop into appropriate chambers...
This should cause slight back pressure and slow and collect water in manifold..which then gets distributed and balanced thru valves to proper flow speed in each chamber...
If set up correctly...should eliminate most of sump tank bubbles...which will then not get to Main tank...
Then (after) just a little more "tweaking and tuning" of Dursos for silence...
Time will help with LR "mini cycle'..
Removing Crabs is a start on DSB "stirring" issues should also allow Micro flora and fauna to regenerate..
Hopefully source water tests out fine, and over time and tank can stabilize at "normal" levels...
Corals come out, Fish are happy, the Sun comes out, and the Wife sings happy songs about how brilliant you are for coming up with this idea in the first place...