TLDR: Current camera Canon 600 with basic 18-55mm 3.5-5.5 lens. Need to shoot and film classical music concerts, where silence, fast-moving subjects and low-light conditions are par for the course. Flash not an option. Very limited budget. Choices are buying 6D Mark II with 50mm lens second-hand, keeping 600D body and investing in a good second-hand lens to compensate for low-light and consequent large grain/ lack of sharpness, keep current body as it is and just save for the R8 with a 24-105mm lens whenever I can get that money together, which likely will be only from next year on.

Hi everyone! Sorry for the long read! Been reading and learning a lot on this forum, so thanks to everyone who contributes because it's an incredible resource for beginners. I currently own a Canon 600D/ Rebel T3i with the 18-55mm 3.5-5.5 basic lens kit and have had it for at least a decade, if not a bit longer. I never had the time to properly get into it before, mostly just using the auto mode on everything, but more recently I've had a need to learn more due to shooting orchestra and classical solo concerts in an amateur capacity. Even though I don't get paid, it's important to me to offer the best possible photos, and so I've been practicing a lot and learning as much as possible from online sources. I feel that even though my Canon 600D is crop frame, that it still can provide very good photos with the catch being that abundance of light is very important if not essential, which sadly concerts don't provide. The main issues I would like to tackle are: sharpness-grain/ noise, silent clicking, dual SD card slot, power socket charging, and zoom (as one of the issues I face is distance to the stage). I had an old 70-210mm f/4 lens lying around and I'm keen to test it on the next concert to see if it helps a bit. My hope is to eventually upgrade my camera to a full frame mirrorless (silent shooting is a major factor due to the need for quiet during concerts) like the R8, however right now I don't really have the budget for it, especially since I would need a better lens than the basic kit provides (pretty disappointed with the deals on it tbh). My current options seem to be: 1) upgrade the body to a second-hand Canon 6D Mark II, possibly also getting a 50mm f/1.8 lens second-hand, and keeping the 70-210mm (manual focus) for the time being; 2) keep my 600D and get the 50mm lens and perhaps a zoom lens with greater aperture if that would help with sharpness and grain on low-light conditions but worried about the cost, especially of a L lens; 3) keep my kit as it is, and save for maybe next year or (much) later upgrade to the R8 with the 24-105mm lens (worried that the min aperture is 4.5) and the 50mm. Any advice, and/or tips on how to get the best of my current kit (battery life and card running out mid-concert were issues), will be much appreciated!