When mentoring in a team that had a couple of junior Devs at a client, I bought all of us ducky's and said that before they ask a question they should try and explain it to their rubber ducky's first and it encouraged them to try on their own first before just asking a question. I found it very good to encourage the behaviour that I don't mind helping them but they must show me where they have tried. In most cases by just trying an approach and not overthinking it they were able to resolve the problem. More than anything it encouraged a way of approaching a problem that the team snapped up really quickly and by the time they asked a question they would have a lot better context about the problem they were trying to resolve. I definitively found it to be very valuable for the team and for myself.

Afaik you will get an instance per module use, so if you use the service in 2 modules you would have 2 instances. If you put the service in its own module (not app. module) then you will have only one instance across the app.

UMLLet is great because you can "type" the design out, which is very fast if your used to the formatting and structure. I also love Freemind and draw.io

I would say a good goal/challenge rather would be to see if you can re-write a system at work by thinking outside the box and use a technology you would like to improve on. For example, if you would like to play around with Node.js and at work, you use C# and a strongly relational database, try re-writing it in Node.js along with different technologies, like using NoSql for example or a Redis Cache, even bring in Docker containers into the mix, just to spice things up. You will be amazed at the things you pick up, and some of those things you will able to apply at work. I found that doing this gives you insight into the systems at work as well as other solutions no one at work has thought about, so basically, a win-win. Also, constrain yourself to the problem the system you are re-writing solves. By having this and this as the only constraint it makes your brain think of really creative ways of solving/approaching the problem differently and gives you a chance to use what you have learned in the new stack very creatively.