For me this is a bit of a tough one. I am a big proponent of going mono. The 585 is a powerful sensor, but it’s still 12 bit. The 2600 is a better sensor in almost every aspect.
From experience i’ve spent quite a bit of time playing with 585s, and honestly I just could never get over the tiny sensor size. Framing targets became difficult with my 100mm refractor. Linearity mode is a big jump for these sensors however. (QHY calls it Linearity, I forget what Touptek calls it). Starvis 2 is some great stuff, but i’m holding out till we get some bigger, better sensors.
I also have had quite a few IMX571s (2600 equivalent) and 16 bit ADC is just far superior. APS-C is just a sweet spot for anything astro as well. You can frame just about anything you want. Again, as a proponent for mono, I would recommend it for use over a color camera.
But I understand your concern for weight, and I think it’s a very valid one. I would weigh your current camera and compare. If you would like the weight of a fully kitted IMX571 mono, I can do that for you when I get home this evening. You should be able to find the full specs for the Minicam 8M online, it’s extremely light, likely lighter than your DSLR. If it was me in your situation, I'd go ahead and upgrade that mount either at the same time, or ahead of time. The used market isn’t all that bad, with patience. It sounds like you’re still getting your feet wet in this hobby, so I understand the hesitation.
If I really had to make the choice, i’d take the 2600MC over a 585 mono, but giving up mono is just as important to me, especially if you have any light pollution.