I'll take your word for it because I'm on the other side of the planet but it just seems incongruent and illogical that replacing a component with another compatible part, mass produced and used all over will get your bike confiscated and invalidated but a totally custom one off with no real world test, certification, or such is fine.
It just seems like bureacracy gone totally out of control. You take bicycles, possibly the pinnacle of appropriate technology, a set of tools can fit in a shoebox and nearly anyone can take apart and fix every last piece, and its super useful. Now incredibly there comes along additional components capable of augmenting and improving the bicycle. Add a multimeter and wirestrippers to your bike toolkit, attach a battery to your frame, replace a wheel or bottom bracket with a motor and any bike becomes an ebike, incredible. But then the law decides to ruin incredible by saying only select pre-approved models can be ridden. And making certain modifications is completely illegal, but dramatically internally modifying is allowed and even insurable.
I hope your pack is safe. Some aspects of the design seem inherently safe like using that many batteries in parallel for a 350 watt load, the peak usage will always be below the continuous rating of the cells. I'm no electrical safety professional but I have taken some workplace safety 101 online website multiple choice certification things and theres some issues i wonder about. Hopefully you've addressed and mitigated all of these concerns already, or plan to as you finalize the build, or get lucky, or are at least aware and prepared if a failure does occur. Here comes more internet stranger critique:I saw nothing about shock and vibration design, no drop tests, no mention of short circuit tests, no thermal measurements taken during a controlled charge/discharge, no mention of arc flash considerations during your assembly, first time use of a new tool that was uncontrollabe/inconsistent during assembly, possibly wasn't connected to a suitable electrical source, an ad-hoc decision to do 3x the number of welds w/no mention of ensuring that didn't overheat the cell.. And that's simple stuff any hobbyist is capable of, nothing compared to controlled third party testing that would be done to certify a commercial pack.
And how many man hours of development and testing do you think go into that commercial battery pack you can buy? Probably more than 2 months. And they're equipped with mutual nda's and proprietary data from manufacturers, vs youtube, online distributors, and some guts of a bosch bms. This was a first time pack build, manufacturers probably destructively tested more packs of each of their models than you have built, and at some point economies of scale and upfront investment give them access to superior manufacturing processes and materials. And because laws get applied equally to every person, consider this project built by another version of you, equally confident and less skilled or lucky, do they too get the official thumbs up to ride it in public?
In the USA if you wanted a 2 kwh pack you can buy them premade, although thats the largest they sell currently, and replace any ebikes battery with it, convert any bike into an ebike, or buy a third party assembled bike with it, or buy from a custom manufacturer that integrates the battery and motor into the frame itself. And needless to say you can put any motor or drive on it. But the idea of licensing, registering, and insuring a bicycle is an unheard of, and its especially word to have anyone inspect your bike or care what parts are on it. Entirely different worlds but it just feels like the European regulations introduce big limitations on the potential.