Limitations
One of the attractive things with Salesforce is that you can add fields all over the place. However as you dig deeper you find limitations, such as the fact that you can't add custom fields to Notes, even though the idea has been around for years (good to see it's in "product review" though). In some ways it's like being back to the good old days of Lotus Notes, where you could create new fields really quickly, though fortunately there is a bit more rigour involved than Notes had when putting them in.If you use a managed package, aka "app" then fortunately you can add fields to their custom objects, which we've made good use of. You can't rename their fields, though here's a tip: use "translations" to override them.
The "classic" interface is two generations behind the latest UI aka "Lightning". The previous generation is VisualForce and you can't change the layout of VisualForce pages, so already things are being locked down for the sake, presumably, of better functionality and preventing people really mucking things up.
Tools
Apart from the standard Salseforce tools I recommend:- The Good Day Sir podcast is a useful source of information, though quite long. It has it's own Slack. (Is that a noun in it's own right?)
- DemandTools has a good range of data manipulation tools and is free for non-profits.
- Jitterbit data loader has some powerful mapping tools for importing data.