Hi -eug,
welcome to the Apple (user-to-user) Community.
Re: iCloud storage: "I cannot downgrade from 2TB to 50GB or something more than 5GB. It seems I'm locked in"
Seems you should have other options:
Apple support's article: Upgrade to iCloud+ and manage your subscription - Apple Support
Eg: downgrade option:
only says to follow the onscreen instructions ... which I assume is what you were doing.
Wondering if this other article is more helpful:
(seems to say you can choose from available plans)
How to switch to a different subscription plan - Apple Support
Else:
You might want to reach out to either iCloud Support, or to Billing and Subscriptions Support:
There's a "Get support - start now" link (online chat, phone, eMail etc)., or a support app you can download,
at the bottom of both websites (links below).
iCloud - Official Apple Support
Subscriptions and Billing - Official Apple Support
All the best :-)