First of all, let me tell you that your english is really really good, so no worries there.
About the low storage, I assume it’s a non-nav unit, right?.
There’s a tutorial that can help you troubleshooting an upgrade from an earlier 3.4 ( Tutorial: Sync 3.4 NON-NAV APIM: Failure to Update to Newer Version Sync 3.4 ) , but you are passed that since you performed the update. Nonetheless, it has really good information about partitioning and how to workaround some issues.
You asked if there’s an issue with your installation method, and this would be my answer: if your unit is operative with the desired version and features, then everything is ok. What you did is what the tutorial says: perform the upgrade in stages, making room for each one of them. And it seems you have done so.
If your unit is not MY20 as your profile says, you can always downgrade to 3.3 and perform a full update (read the tutorial), but again: if the unit is working ok, there’s really no point to that.
But I would advise to read the tutorial, since I believe you could get gracenotes installed afterwards… If you care about gracenotes, that is… It’s just media art…
I think i remember it could be installed afterwards, maybe…