Read the specs it should tell you the max thickness you can go on what ever material you will be using. I think if Quikrete Self Leveling Floor Surfacer is used you can go up to 1" max.

As GN suggests, find the leveler/concrete that allows the thickness you are looking for. And remember to EXTEND the neck of the drain. It still must be free to be used. And your floor finisher is going to have to figure out how to put the face of the drain on TOP of the vinyl. This is no big deal if you are gluing down your vinyl. This is a big, big deal if you wish to float the floor.

In your situation, it is probably cheaper to lay a beautiful sheet vinyl down. It just needs "smooth". It doesn't need flat. A little bit of clean up of the concrete (to remove and bumps or grit) is all a sheet product needs.

Drains are a PITA to deal with...especially with FLOATING floors.

any alternate ideas to SLC? I will need 200+ bags and just not cost effective.

somehow sham and plywood up the low levels first?

Ouch...200 bags = might as well get a new slab! I'm kidding...but not by much...which is why the flooring professionals (who have no vested interest in your choice) will tell you: sheet vinyl or sheet lino. Or you have to go with a VERY flexible (ie. thin) glue down vinyl plank and hope to high-heaven that it works (and the glue doesn't let go leaving planks lifting into space. Knowing full well that you just voided every single part of the warranty by trying to lay glue down vinyl over a large slope like that.

These old time drains in basements are truly some of the most difficult things you can/will face. It takes CONCRETE PROFESSIONALS years to get the technique down. It often involves a plumber because the neck of the drain pipe has to be extended. And the proper cap added to the refitted pipe. And we all know that plumbers (Red Seal in Canada) can bill at $150/hr.

These drains are really hard to figure out. Which is why, many of us shrug our shoulders, shake our heads (to the DIYers) and say, "Sheet vinyl". We then keep shrugging when the DIYer doesn't want to look in that direction.

I've "heard" (never seen) that some flooring/framing guys have been able to lay a sleeper subfloor with shims to deal with changes of roughly 1". They then put in a "trap door" so that the homeowner had access to the drain (for maintenance, etc). Of course you loose 6" - 8" worth of ceiling height by do it...but if you have a 9' ceiling in your basement, it isn't much of an issue.

But a 2.5" variance is quite large. If your upper floors were sloped like that, you would need to bring in a structural engineer to have a look...that's how bad that sort of variance is.

Of course a beautiful epoxy floor would work. Not a big deal for an epoxy expert to deal with that drain.

If, you don't have a height restriction. Leave the drain as is and install a wooden subfloor system with sleepers.

If, you have a height restriction, then begin with grinding down the high spots with 15-25 grit diamonds. It will tear up the cocnrete and any aggregate underneath. It will not be flat.

After you finish roughing it to your desired depth. Use a diamond specified for flattening to begin flattening your surface to the required flatness, recommended by the manufacturer. Most likely 3/16" over 10' diameter. Some patching in the low spots maybe needed.

It will take some time, but will be much more cost effective compared to the expense of 200 bags of leveler - $8000. Ouch!

It will take about 4-5 days with right equipment to prep. Cost of diamond wheels will depend on the type of machine used.