Some leaves are showing small burnt spots and slig...

It looks to me the fist bit of it is just some burning or growing pains for the plant, nothing to crazy and I would not hyper focus on it. I will lilkey outgrow it. As for gnats. their are two methods that works great with little to no side effects. BTi and Nematodes I have used both and I prefer the Nematodes, they are easy for me to get and come in a convenient form. BTi is likely easier to source. Bother a bio control methods. BTi is a spore that when ingested by the larva form of gnats it kills them by reproducing inside them and causing a system failure. Nematodes works almost the same, they are miscropic paractics worms that burrow into the larva and cause system failure. BTi is great in resviours and can be commonly purchased as retail product with the name mosquito dunks. Other options are available. Nematodes can be purchased as a power or other forms. The one I get is called Pot Poppers and they are mini teabags with balls inside them. The balls dissolve and release thousands of the worms who works their way throw the medium and eat the bugs. Yellow sticky traps are great for monitoring but lack a long term solution as they only trap some of the adults not enough to off set the breeding cycle of the pests. Peroxide can help but can also be problematic, it can burn roots if not diluted and does not have a higher kill rate at low concentrations. It can also kill off good microbe life in the soil and allow other opportunistic ones to move in easier. Applying a thick layer on the soil base can help prevent them from burrowing but it notmally need to be very fine like sand. and that adds a bunch of weight on the soilline compacting it. I have yet to use Beauveria, it looks like a good product, this alone might be all that's needed to kill the larva cycle. I would apply this in the soil vs on the leaves. It will take about a week to fully eradicate em if done correctly. Use sticky traps to monitor their populations.