Mixing new Marco rock w/ very old live rock (w/tap water use for 2 years)????

I'm restarting a 34 gallon Red Sea cube. It is very old live rock that has not been cared for super well for the last 3--4 years. The rock is probably 15-20 years old but has been heated and maintained at least one fish the entire time. Topoff water has been city tap water for about 2 year. There was a crash 3-4 years ago prior to using tap water and ONLY my clown survived. The water has not been super clean for several years but clean enough to keep the clown alive.

I'm wanting to start this tank fresh but keep my clown alive. I'm going to be switching to a Marco rock landscape and I eventually (soon than later) want to get rid of ALL of the current live rock.

When I do add the Marco rock landscape, should I keep some of my live rock in the tank to help seed the tank and keep the clown alive?

I've heard that things can get absorbed into live rock (phosphates, bad metals, etc.) over time and these can leach back into the water. Is there a risk of my new Marco rock absorbing bad stuff (bad metals, phosphate, etc.) from my old rock through the water?????

If you think I should use some live rock in this tank, how many lbs would you use? Would you break it up and put it in the back compartment or leave it I the display????

I've heard you can scrape rock with Coraline into a tank with new rock to "seed" the new rock with Coraline. Think this is true???

Any other thought, tips, etc. appreciated???
Have you thought about using something like Caribsea’s Life Rock (instead of dry Marco) that is infused with bacteria?

I’d be concerned with replacing all the live rock at once with dry. Yes, old rock can leach bad things like phosphates. So, I’d also be a little concerned with keeping that rock in to seed the new rock, especially since you’ve been using tap water. Maybe you can get a report from your water company on what’s in the tap water that might help you/us understand potential issues with the tap water.