For some years I have been using a Nikon D500 with a Sigma 100-400 zoom and have been very happy with the results. However, at age 77 this combo is getting too heavy to carry (I travel a lot and spend part of each year in Malaysia where I am often shooting in forest interiors at high ISO levels). I would value advice on any lightweight combo that would give me similar performance at a lesser weight (my current combo weighs about 2 kg). I photograph mostly birds, flowers and insects (especially dragonflies). Is my best bet an OM1 system?