You're on the right track. I do a ton of this in my day job as an AWS TAM for an MSP**

RE: Reduce delay in cost awareness**: Teams should know about unexpected costs as soon as possible.

AWS Budgets allows gives you a few options here. You can not only set up spend-based alert thresholds, which could go to your email address, but I would use a distro. There are also some newer AI-oriented anomaly detection functions also.

RE: Identify wasted resources easily: Instead of playing a guessing game, we should pinpoint which resources are consuming costs unnecessarily.

Two options jump out at me here:
#1 The AWS Trusted Advisor report. This is an in-depth spreadsheet sent out to clients with enterprise-grade support levels, meaning partner-led support with a 3rd party accredited support provider, AWS OnRamp, or AWS Enterprise-level support tiers.

This report tells you exactly which servers are grossly oversized and other components that could be optimized, archived, or downscaled to save money.

#2 There's a module for an opensource tool called PowerPipe called "Thrifty", which can assist there also.

The AWS console under Cost Explorer will also show you your available instance reservation options and/or savings plans.

RE:Find top cost contributors: Generate a report showing the top 5-10 resources that are contributing to high costs.

I would highly encourage you to learn how AWS Cost Allocation tags work. There are defaults which will give you what you're asking for, but there's some much more advanced and granular options with what are called "User Generated" Cost allocation tags.

RE: Questions I Need Help With Should I build a unified multi-cloud system, or is that too ambitious for a beginner?

If you are new to cloud infrastructure, I would definitely not start off with a multi-cloud environment. Even if you weren't a beginner, there needs to be a very compelling reason to chase "best in breed" services across a provider boundary. This is almost always more hassle than it's worth.