TL;DR Yes, use Node.js. The longer answer is _it depends_.
Here are some things to consider:
1. Existing Infrastructure and Expertise: Since you already have projects running on NextJS and Node.js, your team likely has expertise in JavaScript/Node.js. This familiarity can be a significant advantage in terms of development speed and ease of maintenance.
2. Cost-Effectiveness: Node.js developers are relatively abundant, which might help in finding talent within your budget constraints. Moreover, the JavaScript ecosystem is vast and has numerous free, open-source tools and libraries, which can be cost-effective for a startup.
3. Scalability: Node.js is known for its non-blocking I/O model, which can handle numerous simultaneous connections, making it a good fit for applications that need to support a large number of concurrent users. However, the scalability also depends on your application's architecture, database design, and other factors.
4. Performance: For IO-bound tasks, Node.js performs exceptionally well. However, for CPU-intensive tasks, it might not be the best choice. Since you are a startup aiming to scale from ~25K to ~100K users, Node.js can handle such traffic effectively, provided the application is well architected.
5. Ecosystem and Community Support: Node.js has a large and active community. This means a wealth of libraries, tools, and frameworks are available, along with strong community support for problem-solving.
6. Integration with Existing Projects: If your existing projects in Node.js and NextJS are functioning well and are expected to be part of the new project, using Node.js can ensure seamless integration.
Node.js is still a very relevant and viable option for production code, especially for a startup with existing Node.js and JavaScript expertise, looking for a cost-effective solution that scales well. It's essential to also consider the specific requirements of your project and possibly consult with your technical team or a software architect to make the best decision.