Multiday lab courses in classic chemistry
Beside asking upfront the research groups, there are summer schools in normal (non-covid 19) circumstances.  For example Oxford's Chemistry Summer School, or ACS' Green Chemistry Summer School.  Given your account name, I speculate you reside in Europe; if so, a suitable directories page not only about core-chemistry may be  www.summerschoolsineurope.eu.  Sadly, because of the current pandemic, many scientific events are cancelled, though (example).  Still, references of courses in the past are retained (example), so you could find help by the former organizers as well.
Maybe you are an outlier by age in comparison to the «typical attendee».  However, the experience you gained after your master thesis, both in terms of chemistry under the hood as well as e.g., project and data management, may be of great benefit to the class providing additional perspectives.  In particular, I think about planning the experiments in the smaller groups, or discussing the results.  Equally, you may become a role model for younger participants in the way you choose to stay continuously engaged in learning.