Regarding your comment about mindfulness and CBT – did you know that a large component of CBT is mindfulness? That is the reason why they do not find a big difference in some of the studies.

Also, do you know what mindfulness is? It goes way beyond simple relaxation – it is also about getting to know your mind and developing self-knowledge, etc. If you had a look at the links I posted in a different comment here, you might get a better idea about what mindfulness is, etc., and you will also see that the brain also changes (in positive ways) with mindfulness practices.
Anyway, below are some meta-analyses:

Blanck, P., et al. (2018). Effects of mindfulness exercises as stand-alone intervention on symptoms of anxiety and depression: Systematic review and meta-analysis. Behaviour research and therapy, 102, 25-35.

Scott-Sheldon, L. A., et al. (2019). Mindfulness-Based Interventions for Adults with Cardiovascular Disease: A Systematic Review and Meta-Analysis. Annals of Behavioral Medicine.

Wang, Y. Y., et al. (2018). Mindfulness-based interventions for insomnia: a meta-analysis of randomized controlled trials. Behavioral sleep medicine, 1-9.

Zou, L., et al. (2018). A systematic review with meta-analysis of mindful exercises on rehabilitative outcomes among poststroke patients. Archives of physical medicine and rehabilitation, 99(11), 2355-2364.

Goldberg, S. B., et al. (2018). Mindfulness-based interventions for psychiatric disorders: A systematic review and meta-analysis. Clinical psychology review, 59, 52-60.

Carsley, D., Khoury, B., & Heath, N. L. (2018). Effectiveness of mindfulness interventions for mental health in schools: A comprehensive meta-analysis. Mindfulness, 9(3), 693-707.

Carrière, K., et al. (2018). Mindfulness‐based interventions for weight loss: a systematic review and meta‐analysis. Obesity Reviews, 19(2), 164-177.

Dunning, D. L., et al. (2019). Research Review: The effects of mindfulness‐based interventions on cognition and mental health in children and adolescents–a meta‐analysis of randomized controlled trials. Journal of Child Psychology and Psychiatry, 60(3), 244-258.

Borquist-Conlon, D. S., et al. (2019). Mindfulness-based interventions for youth with anxiety: A systematic review and meta-analysis. Research on Social Work Practice, 29(2), 195-205.

Van Driel, C. M., et al. (2019). Mindfulness, cognitive behavioural and behaviour‐based therapy for natural and treatment‐induced menopausal symptoms: a systematic review and meta‐analysis. BJOG: An International Journal of Obstetrics & Gynaecology, 126(3), 330-339.

Simpson, R., et al. (2019). Mindfulness-based interventions for mental well-being among people with multiple sclerosis: a systematic review and meta-analysis of randomised controlled trials. Journal of Neurology, Neurosurgery & Psychiatry, 90(9), 1051-1058.

DiRenzo, D., et al. (2018). Systematic review and meta-analysis: mindfulness-based interventions for rheumatoid arthritis. Current rheumatology reports, 20(12), 75.

Rusch, H. L., et al. (2018). The effect of mindfulness meditation on sleep quality: a systematic review and meta‐analysis of randomized controlled trials. Annals of the New York Academy of Sciences.

Li, J., et al. (2018). Mindful exercise versus non-mindful exercise for schizophrenia: A systematic review and meta-analysis of randomized controlled trials. Complementary therapies in clinical practice, 32, 17-24.

Khoo, E. L., et al. (2019). Comparative evaluation of group-based mindfulness-based stress reduction and cognitive behavioural therapy for the treatment and management of chronic pain: A systematic review and network meta-analysis. Evidence-based mental health, 22(1), 26-35.