Here's some suggested minumums for insulation.
Here's just some guesses on things on things you may be dealing with that I've found on the dozens of 100 plus year old houses I've remodeled.
Ballon constrution, so the walls are open from the crawl space or basment to the attic. The walls need to be blocked at the bottom to cut off the air flow.
No one has ever air sealed all the holes where wiring and plumbing were run in the bottom plates. They need to be sealed with expanding foam.
Someone may have installed replacement windows but never removed the trim around the window to insulate the huge holes on the sides. In most cases there's about a 4 to 6" wide hole direct to the outside where the old window weights were, and about a 2" at least gap under the window and also the top.
No one has ever sealed up where the lights, ceiling fans, vent pipes, and wiring were run in the attic. A simple sealing up with some foam will fix that.
Just because the home is old and has never had any vents does not make it a good idea. Lots of old homes had no insulation at all, no soffit vents and no roof vents. But they also never had HVAC to heat and cool the home, now you do.
All of these things will not cost, they will save you money and there all simple easy DIY jobs.