It’s a niche car so options are limited. I would suggest these are the closest equivalents but the price increase is big!
If equivalent means Mercedes then:
AMG GT 63 - not an estate but a big car, and closest in concept. Big step up in price.
If equivalent means shooting brake then:
Panamera Turbo SportTurismo - not an AMG but beautifully engineered. Big step up in price.
I would suggest that in truth the Panamera SportTurismo is the closest of all in concept and is a stunning car.
I spent a week with the Sport Turismo before I bought the GT. It wasn't for me, its heavy and it feels it (not to say the GT is light) and isn't worth the 20k premium over the more capable GT63s. The base car is poverty spec, and for it to be the car it should be its over £170k new. These will be rare in the future as few have found homes in the UK due to the price point, its Bentayga/Contienental/GTC 4 Lusso type money.
The boot isn't that practical in either of these cars compared to the CLS. The GT boot is big but awkward. The Panamera ST is ok but the opening is terrible, no good for kiddie prams and multiple large cases.
I loved my CLS350SB, I will have another as a daily when they get a little cheaper. I understand sentiment regarding mortgage payments and owning the car, but your house isn't a commodity, you always need somewhere to live. I have been in your situation and I did borrow the £10k extra I needed to get exactly what I wanted and I never regretted it once, ended up owning the car long enough (10 years) it more than offset the cost. I would stretch the funds if it allows and go buy Nito's CLS63s, its the bargain of the century, low miles, and you will love it for years to come becuase it's what you are looking for.