Nopy is definitely a great service. For me it's faster than mega and workupload (which has been first and second choice for years) but I haven't seen it taking full usage of my 350mbit (Haven't tried nopy for a couple of months, since before the upgrade) so I tend to download using mega together with my Real-Debrid account.
Having worked as a sysadmin myself, I know how expensive high-bandwidth servers can be, especially with ddos mitigation and a proper firewall. I hope (and think) you have plans on how to get something back for your hard work, without sacrificing user privacy, download speed, or spamming users with ads. Some ads (like on f95zone) are perfectly acceptable though. I doubt that relying on donations is enough to keep Nopy up in the long run.
Keep up the good work!
Edit: Just read your response to the issue of covering the server bills. Glad to hear that you're getting enough donations at the moment