I've no idea about the electrics, but either do a search on the forum as someone has probably posted a similar issue at some point, or make a new thread specific to the wiring issue - someone will more likely see the post and reply to your issue. When I started I had an old berco which had a dodgy thermostat. I could easily bypass the temp controller and made a standalone controller (think Foxbat wrote a thread on that). I quickly moved to a Digiboil (very similar to a Klarstein) which has 2 heat settings (900w and 1500W - can combine both for rapid heating). If you can't get your boiler sorted, them long term definitely worth looking into something that can give you more control (if you plan to continue AG brewing).
If you haven't already done so, download a brewing app (I use Brewfather) and play with a recipe. I would start by inputting the ingredients you have just used and see how the figures it produces compare to those from the kit instructions. I'd also suggest that by using a brewing app, you can take into account your rigorous boil and reduce your boil time (you'll find a thread on here with lots of opinions about boil lengths). The brewfather app lets you change the boil off rate (you may have an indication of this from your brewday?). Using the app you can take into account the rigorous boil and get revised mash and sparge volumes. As previously noted, you can also post-dilute your wort after the boil if it's gone above your target OG (naturally that may have an effect on bitterness). By reducing your boil time you will also need to change your hop schedule. Again, the app takes this into account so you can still get your desired IBUs.
As you say, it's quite a steep curve, but fun. Don't hesitate to ask a question on here as everyone's very friendly...and we've all started with similar issues. Happily, unless you make a massive mistake (I've inadvertently contaminated a brew with rain water....not a good result!!) you'll get good tasting beer. I've been doing AG for a year now and haven't moved past the initial stages. I've yet to think about changing my water profile or even looking at the mash pH (that has an effect on efficiency I believe). There is lots to learn, but as I'm short on time and very happy with my beers to date, I'll take the next steps once I find time. john