It's important to understand: Aternos is not a bad service. They do an incredible job providing free hosting to millions of players around the world. But the free model has its price.
Aternos earns money from advertising. Every banner in the control panel and every wait in the start-up queue is what keeps the service running. Server hardware, electricity and traffic all cost money, and ads cover only part of the costs.
Because of this, Aternos has to save on resources. Hundreds of servers run on one physical machine at the same time. CPU time is shared between all of them, memory is limited, and servers are shut down when idle to free resources for others.
The limits on plugins and mods are understandable too: some plugins use too many resources and can affect neighbouring servers. DDoS protection is an expensive service that can't be provided for free at Aternos's scale.
Paid hosting works differently. You pay for dedicated resources (CPU, memory and disk) that belong only to your server. Nobody shares them with you, there are no queues, no limits on plugins, and the server runs 24/7 whether there are players on it or not.
From €1.99 per month you get a completely different level of hosting. And if your server has outgrown Aternos, that's a good sign: it means you're ready for the next step.