Best addition to Sigma 18-35 for 6K?

Which zoom lens?

Or should I better get primes?

Mainly scenic work.

Depends on what you are shooting and your budget. I have the Sigma photo 50-100 which mates well with my 18-35. But it's focus breathing is crazy. Lenses are some thing you might keep a life time so don't rush into your next lens. Think really hard about what you want to shoot and how you want to shoot it. There are a mile of comparison reviews on YT that you might want to look at.

I think the Sigma glass you have is a great and very sharp general-purpose lens. A 50mm or 85mm lens is an important buy depending on how close you like to get.

There are many people offloading their L Series EF Canon lenses for the new RF mount, this makes getting those L Series lenses a great second-hand deal. Even getting a broken EF lens and spending 300 to 500 to get it repaired is a good deal (to me). If you don't mind taking apart lenses some of the issues like fungus are repairable at home. Purists want perfect gear but I appreciate the old time mentality of fixing your own gear if you can.

yYou might want to get some vintage lenses.

I recently picked up a set of Super Takumar lenses 120$ with M42 mount (estate sale) and bought some adapters from amazon. I have to tell you the look is amazing and hard to describe, but I will try..., It's a "buttery" image that is both soft and contrast-y at the same time. The 24mm f3.5 is an amazing value. These kind of lenses make the camera output look less digital IMO but your experience and eye may be different. These lenses don't look too impressive to clients if that's your game, but the image speaks for itself.

When I first got my camera I went to a pawn shop and found a Mamiya/Sekor 50mm F2 (the non- radioactive one) for 10$ in a bin and it's one of my favorite lenses for faces.

Hope that helps

I'd suggest Zeiss C/Y lenses, like the 50mm f1.7 or the 85mm f2.0. They match pretty well with the Sigma and are not too expensive. The 50mm f1.4 and the 85mm f1.4 will cost you much more (and the latter is heavy). I love those Takumars too, but they have quite different characteristics.

I'd suggest Zeiss C/Y lenses, like the 50mm f1.7 or the 85mm f2.0. They match pretty well with the Sigma and are not too expensive.

Some of the C/Y lenses have actually gotten pricey in recent years due to high demand from cinematographers. You could also look into the later Zeiss ZE lenses, which are sometimes cheaper now, at least this was the case with the 35mm 1.4 Distagon when I was shopping for one a year or two ago. Advantage is that it already comes in EF mount so no adapter needed, and you may be able to find one that was cine-modded by Duclos. I have a ZF.2 (Nikon mount) 35/1.4 that is partially cine-modded and wow, what a lens. But you'd want one of the longer ones to complement your Sigma 18-35.

Well, the 35mm f1.4 was never cheap. But with todays cameras and their low-light capabilities, the two I suggested will not cost him an arm and a leg. They can be modified for EF-mount with Leitax adapters.

Everybody is craving for the faster ones and they went up badly.

I like the Sigma 18-35mm and use it a lot. But in time, I will replace it with a DZO Pictor set of 14-30, 20-55, and 50-125 parfocal and use it on the P6K and UMP 4.6K G2.

This DZO Pictor T2.8 set at $8,200 (PL & EF) looks like it’s what you might go for:

https://www.bhphotovideo.com/c/product/1701860-REG

But you might want to also consider having a similar range in one zoom with this $10,000 Fujinon 20-120mm PL T3.5:

https://www.bhphotovideo.com/c/product/1322733-REG

Actually I’m wrong since you would still need a wider zoom with the Fujinon. I use the Tokina 11-20 which works well but increases the cost. The DZO Pictor set is less money and that’s always an important consideration. Not to mention the cool white finish.

I'm not sure he's aiming at that level. But without further detail, who knows?

another 2 cents:
I'd go with either the Fuginon mentioned by Rick or the Tokina line. I've used the Tokina 25-75 T2.9 as a rental with the URSA 12K and it was nice... even considering it again to purchase since Duclose has it on sale.
https://www.ducloslenses.com/collections/tokina-cinema-atx-zooms/products/tokina2575
(I was considering it last year but got a Zeiss Otus 28mm instead... and glad I did... fantastic lens on the 12K.)

I have the older Tokina 11-16 PL cine and it still works great... but can show some CA (got it before the 11-20.) The 25-75 does not have any noticeable CA (and has minimum breathing, none noticeable in the shoot I did)... great image quality from that lens.