As someone who's been in fledgling mode for decades, allow me to offer a few bits of advice.
P-G is a friendly and helpful place, full of experts in many fields, but posting under an alias likely won't draw much response. Personally, I'd rather give advice to a genuine person so that I may gain some understanding of the ecology of your region, objectives and methodology.
Pearl farmers tend to fiercely guard their technology, especially with the nuances of their individual sites, though much is common knowledge. I saw a video a short while ago, where Mr. J.C. Branellac of Jewelmer made an important point. His advice was to "aim high, or you'll never get there". That means three things... 1- Competition. 2- Viability and 3- Quality. As a FW farmer, you will need to compete with China, who have developed hybrids to maximize all three of my points. Likewise you'll need to stock, rear and produce in the face land use issues, taxation and environmental controls etc. And lastly, gem quality from your target species.
Prior to putting the cart before the horse, you will need to assess your mussels. You have identified the Flood Plain Mussel (Velesunio ambiguous) in the Hyriidae family as your target. Do you find natural pearls in this species? Do you observe shell anomalies and parasites? I gather you have some savvy about the physiology of mollusks, but do you intimately understand their anatomy and metabolic functions? If the answer is no, then it's virtually impossible to help you.
I say this, because even the most advanced technician can be a fish out of water when they are away from their own farms. I am aware of 100% failures by skilled grafters working with foreign species. Farmers don't wish to send anyone on wild goose chases by providing misleading or non-applicable advice mainly because they know the cost of failure. Don't become downhearted if only a few speak up, after all, at the end of the day it's entirely up to you to find your own ways that work. Failures are lessons learned. Rising above failures can but not always mean success. Many technicians can produce beautiful pearls that are in no way viable. Even among the most successful farmers, the profit margins are miniscule.
In the meanwhile, I cannot emphasize strongly enough to observe and document the nacreous processes of your shell stock. Collect loose pearls and juvenile shells. By whatever means, get those samples to a reputable lab that knows their stuff. Obtain a report about the structures of your samples. This would include stratigraphic electron microscopy, radiography, chemical analysis and growth rates. Calcite is your enemy. If your species generates high concentrations calcite at an early age, you are dead in the water. You know the old saying, you cannot make a silk purse out of a sow's ear. You only want to hear one result from your lab... terraced aragonite... nothing more.
You mentioned gonads. The gonads in mytilids are nothing like the gonads of a pterioid. Pterioids have gonads as singular organs with supporting tissues. To the best of my knowledge, pearls are grafted adjacent to these organs, not themselves. Mussels on the other hand, have tens of thousands gonoducts which share the same space as the mantle tissues. Pre-graft conditioning is necessary. Acquiescence will not occur in the presence of gammets (reproductive cells) or during quiescence (hibernation of a kind) or reversed mineralization (low salinity or food).
You must find a way to non-fatally penetrate the micro thin layer of the inner epithelium without damaging the outer whilst avoiding the ducts. For your sake, I hope you have a sharp eye, a steady hand and precision tools.
Not even the most erudite can tell what those windows of opportunity are in the absence of research data. Although he passed on years before I was born, Ed Ricketts said it best. Observation, speculation then replication. Don't sweat the latter yet unless you've established the formers.
