Advice on expanding storage in starter homelab/media server

I've been slowly fiddling around with setting up a little homelab and media server for the last few months. As a web developer, I've always wanted to learn a bit more of the infrastructure side of things, hence the homelab part. The deteriorating quality of major streaming services finally pushed me to set up a media server as well.

Right now, my setup is very basic. I've been using an old repurposed office laptop. It's a simple dell latitude 5540 I got ridiculously cheap due to it's barely usable crusty keyboard, but since I mainly SSH into it that's not really an issue. I formatted it, doubled the ram, and installed the latest stable Debian release. (Headless)

After that, I chose to install yams which was recommended here. Definitely saved a lot of time there! Finally, I added an old unisex raspberry pi I had lying around. The idea is that it's the only part of the setup that is on 24/7, since it has an almost negligible footprint. Whenever I want the main server running, I SSH into the raspberry and use wakeonLAN to start the main server. I'm probably gonna make a tiny web interface for that soon.

Now on to the part I need advice for. The laptop and attached HD are quickly running out of space. I know just slapping on extra hard drives has a limit, and am vaguely aware of things like unraid existing, but am a bit overwhelmed right now with all the information and options in this space.

Does anyone have some advice on something I can tackle for a reasonable amount of work/budget? Something basic, but with the possibility of expansion in the future?

Any other tips on where to go next in general are of course also appreciated. (On that note, I'm right now not opening up the server to ingress from outside. I only interact with it on the home network, as I primarily work from home)