Totally get the $ concern. I'm saving up myself for a standing desk one day as they allow greater adjustability in the long run.

If you have a chair already, then use what you got. The best idea is to sit-stand-sit-stand and move around as much as possible during WFH. Maybe look into a keyboard tray like this one. It'll allow for more relaxed arms/shoulders because your laptop on that surface will require you to have what I call T-rex arms, typing away in an awkward position. Also, elevate your laptop. It'll help your neck/back.

Thoroughly intrigued by your cycling idea 😅. Don't go ham at the cycling while working as you don't want to cause more harm than good. Especially for your upper back/neck area. I watch this YouTube channel GCN - they have great tips for cycling, cyclists. Here's an example.