5M - fatigued sometimes and has some musculoskeletal issues

My 5-year-old son (21.5 kg, 3’7.5”, otherwise healthy) has had hard/stiff swelling and limited flexion in two fingers for about 6–7 months. He never complains, but I noticed him guarding the hand. He cannot make a full fist, and flexing beyond a certain point becomes uncomfortable/painful.
He also has an intermittent guarding/abnormal gait. Earlier imaging showed mild tenosynovitis and later a very mild tibiotalar ankle effusion.
Four months ago, labs showed positive ANA and mildly positive RNP (1.6). Hand MRI showed trace flexor tendon-sheath fluid/very mild tenosynovitis, minimal STIR signal and very faint synovial enhancement, but no clear active synovitis. Rheumatology did a thorough joint exam and found no obvious active inflammation, so they felt this could be either an early or resolving inflammatory process and decided to monitor.
He has now been on scheduled naproxen for about 2 months. We’ve had a few short refill gaps (1–2 days and once about 4 days), and each time his finger stiffness/discomfort and guarding gait become more noticeable. On naproxen, his walking improves and I can flex his fingers much more easily and without the same discomfort.
We repeated labs 3 days ago for another reason. ANA/RNP were not repeated. ESR is 10 (lab range 0-10; it was 8 four months ago), CRP <0.5, and CMP is normal.
One additional concern is that sometimes he seems unusually fatigued/low-energy for periods of 2–3 weeks. He still eats normally and plays with us, but I can tell that his energy is lower than usual.
For pediatric rheumatologists/physicians: How significant is the repeated improvement on naproxen and worsening when it’s stopped? Can a low-grade/localized inflammatory process still be present with normal ESR, CRP and CMP and such minimal MRI findings?
Also, with these reassuring labs, is there anything more serious/dangerous that should still be considered because of the persistent finger/ankle symptoms and intermittent fatigue, or are these labs generally reassuring against that?
Finally, how long can a child typically remain on scheduled naproxen? If symptoms consistently return off it, when would treatment aimed at the underlying cause (such as a DMARD) be considered rather than continuing daily NSAIDs?