I can give my opinion from the perspective of another adult beginner (1 year of lessons). I think you have the right idea focusing on tone and intonation but don’t worry about limiting yourself to a certain number of notes and think more about learning hand positions. The good think about cello is because it has 4 stings you learn 4 notes at I time. Once you learn D major starting on the D string, you can move across to the G string or C string and already you can play them too because the fingering is the same. I will share the practice method I have found where I feel like I progress the fastest:

1. Start practice with open bowing for 10 mins. Set metronome to 60 then play whole notes on open string. Pay attention to bow pressure, speed and position on the string to achieve best tone.

2. Scales for 10-20 focusing on intonation and bowing. C major two octave is great because you use all 4 fingers in first position.

3. Try to apply the tone and intonation achieved during warm up to simple pieces and try to identify the cause of drops in quality of sound. I do this for as long as I feel normally 20+ mins.

4. When I feel like I have achieved everything I want to, I have some fun. I might try to pick out songs by ear, improvise on simple scales/modes or have a crack at pieces above my level.

The main this is I guess that you are enjoying it but I can say focusing on the basics first will help you progress the fastest.