This comment would be 60 times more helpful if in addition to your strong opinion on the failures of learning with Duolingo it’d supply some of the good alternatives.
As someone learning Japanese I'm really appreciating tools built for JP specifically: Renshuu and Wanikani. Both use SRS (same as duolingo) but spend a considerable amount of time actually teaching the grammar and nuances, they both avoid starting from everyday phrases like "I would like sushi" to instead build a foundation first, and many other little things that make it a much nicer experience than Duolingo who's trying to use a very generic approach that maximises small term satisfaction in exchange for painful long term learning.
You will end up paying much more for your services, along with spending a ton of time maintaining it (and if you don't, you will probably find yourself on the end of a 0-day hack sometime).
In Northern/Western Europe, where power costs around €0.3/kWh on average, just the power consumption of a simple 4 bay NAS will cost you almost as much as buying Google Drive / OneDrive / iCloud / Dropbox / Jottacloud / Whatever.
A simple Synology 4 bay NAS like a DS923+ with 4 x 4TB Seagate Ironwolf drives will use between 150 kWh and 300 kWh per year (100% idle vs 100% active, so somewhere in between), which will cost you between €45 and €90 per year, and that's power alone. Factoring in the cost of the hardware will probably double that (over a 5 year period).
It's cheaper (and easier) to use public cloud, and then use something like Cryptomator (https://cryptomator.org/) to encrypt data before uploading it. That way you get the best of both worlds, privacy without any of the sysadm tasks.
Edit: I'll just add, as you grow older, you come to realize that time is a finite resource, and while money may seem like it is finite, you can always make more money.
Don't spend your time hunched over servers. Spend it doing things you love with people that matter to you. Eventually those people won't be there anymore, and the memories you make with those people will matter far more to you in 20 years, than the €20/month you paid for cloud services.
Google One for 10TB is 274,99€/mo (at least in my country) so you'd make the entire nas price and subscription cost within a few months, let alone years.
There just aren't compelling public cloud for large sizes (My NAS is 30TB capacity and I'm using 18 right now) and even if you go the more complex loops with like S3 and whatnot you still get billed more than it's worth. Public cloud is meant for public files, there's a lot of costs you're paying for stuff you don't need like being fast to access from everywhere.