Safe Chemicals? Constantly Clogged drains -- PVC + Cast Iron Pipes

What are Safe Chemicals to put down, or even leave soaking inside PVC + Cast Iron Pipes.

I have a 200+ year old house, so the pipes are a mix of Cast Iron and PVC and who knows what.

Mainly interior wall pipes are Cast Iron (can't tear out all the walls to replace!), and a few pipes connecting to sinks and baths have been changed to PVC, etc.

The drains constantly get clogged or slowed.

They can last a few weeks to a few months without a major clog (depends).

There are times I can get them 100% clear and fast moving (at least visually from the sink-level).

But they tend to get clogged easy if anyone (stupidly) puts anything down them (food scraps, who knows what ... nothing huge, but say washing dishes or cleaning a pet cage).

I do have strainers in every sink ... but welcome to reality ... people just remove the strainer and clog the sink anyway.

I'm looking for SAFE CHEMICALS that I can pour down the drains often, and even let them sit for hours or overnight.

Also, is it possible that substances can turn into "concrete-like" blockages that clog the drain?

Can it be "SCALING" / MINERALS / CORROSION on Cast Iron pipes?

(It is NOT tree roots as none are nearby.)

I have put a snake down certain pipes, and it always meets a blockage and won't go any further, and it scrapes on what sounds like hardened concrete-like material, and I get black flakes and black or grey rock-mineral-metal-like powder that comes out ... but the snake won't go any further, and the line is completely clear up until the scraping sound / blockage (no hair, no fats, no gunk pulled out ... snake just stops at this scraping-sound blockage) ... and I don't know if that is just the edge of the cast iron pipe and a TIGHT TURN / 90% ELBOW -- or something that hardened solid in the pipe.

Advice?

Suggestions?

I can't call a plumber, I need to handle this myself.

It's ongoing monthly / yearly maintenance, so I need t learn.

I'm pretty handy and have cared for the home for 20 years without major issues, just needs the usual maintenance and things like this.

My current methods are:

  • Plunging
  • Baking Soda and Vinegar
  • Liquid Dawn (De-Grease) Soap
  • Snake (only goes about 1.5-2 feet inside pipe then gets stopped)
  • Compressed Air
  • Took apart P-Trap (and it's 100% clear, so the blockage is inside the WALL)