Your cathode needs to be significantly smaller. I run 10ma and found I needed to go to a 24 gauge copper wire to give me a little more ease in adjusting cell voltage (about 1" submerged) otherwise its a bit too touchy. Usually I have about 0.5" to 0.75" submerged. At 1" submerged, that comes to about 0.065 square inches of wire surface exposed to the cell (about 1.6 square millimeter and yeah I said millimeters). Yeah. tiny. That "flag" cathode you have is HUGE. Its no wonder you have issues getting the cell voltage up over 10V.
With that converter, at 10ma, 28 V out, 5VIn you should only be using about 60ma assuming 90% converter efficiency. 2amps at 5V is HUGE but then, these phone chargers are tiny.
That constant current/constant voltage unit has been discussed before and its not nearly accurate enough to make good quality Colloidal Silver (what I recall from the outcome of the discussion). You really need a home-built current limiter. This could be as simple as an LM317 voltage regulator with a resistor (or potentiometer) following it and its very stable though it requires 3V minimum above your cell voltage (just saying) as operating headroom.
I'm about to do the same thing with that exact same boost converter module (been thinking about it for a while but haven't gotten around to doing it yet). I'm going to power it from one of those little Apple IPhone chargers (the ones that look like maybe a 2cm cube). Those are 1 amp and thats way overkill but TINY.
You can add the reducing agent at the beginning of the run so long as you keep the cell temperature up over maybe 120F so the reducing agent reduces somewhat quickly, not hours like it does closer to room temp. Then, you can just continuously produce and raise the PPM to whatever you want because you never have a situation where you're over the solubility limit for silver oxide dissolved in water during the run (what I do) as long as you add enough reducing agent to reduce whatever total PPM your target is (plus a little extra for "insurance"). The added benefit is that when the run is done, you're fully reduced (I let the solution sit maybe 30 minutes to make sure every last bit of IS has been reduced before I gel cap - it does take a little time for the reducing agent to do its thing).
Honey is NOT a good reducing agent. First off, its NOT consistent regardless what you think and secondly, its dirty. There's even a bit of peroxide in it which I'm not so sure is a good idea as an ingredient in a reducing agent. Light clear corn syrup is very clean/consistent. Maltodextrin (as long as you buy it from a reputable source and if you do, try to get one specific DE number so you can compute how much you really need to use), even glucose are clean and consistent.
Maltodextrin is long chains of glucose molecules each hanging on the tail of the next to put it simply. Only the "head" at the very end of the chain can reduce. The DE number (if you can get single DE maltodextrin which at least in the US at the consumer level is quite difficult - its all mixed bag) refers to its dextrose equivalency. DE is usually a number ranging from 2 or 3 up to 20 or even a little higher though usually above 20 its referred to as glucose syrup and yeah, its a liquid over 20 usually. If you can find it you want a single DE maltodextrin because then you can compute exactly how much you need for reduction. The number is a percentage you divide the weight in glucose you need to completely reduce your IS to Colloidal Silver by as a fraction which tells you how much maltodextrin you need. For instance, if you need 0.1g of glucose (dextrose - same sugar, 2 different names) and you have maltodextrin with a DE of lets say 5 (a.k.a 5% of the reducing power of glucose) you'd compute (0.1g/0.05) = 2grams of maltodextrin. As you can see, its advantageous to go to a higher DE number but the lower the number, the better a stabilizer the maltodextrin is. As I said though, in the US at the consumer level we don't have a choice as its "mixed" bag so all we can do is assume a DE number of maybe 3 as a worst case meaning, again for that 0.1g of glucose example, we'd need to use 3.34g of our unknown maltodextrin.
Just so its said, the solubility of silver oxide in water at room temp (thats 75F, not lower) is a hair over 20PPM. At 150F its about double so about 40PPM.
I do my runs at 150F so the reducer I add at the beginning reduces quickly (maybe 5-10 minutes) so I never have a situation where I'm exceeding the solubility limit for what hasn't been reduced yet.