Replacing large steel pipes going to/from hot water boiler to cast iron radiators with PEX

I am finishing the basement in our 100yo 2-story bungalow in St. Paul, MN. The basement's ceilings (cement floor to bottom of exposed ceiling joists) are 7ft high or less so every inch of headroom that can be gained is a big deal. I have cleaned, primed & painted the exposed bottoms & both sides of the basement ceiling/first story 2" x 10" floor joists, rim joist end cavities & bottoms of the 1" thick rough-sawn sheathing boards under the hardwood flooring on the first story, as well as all the electrical wires & natural gas lines running thru them. Everything looks great, but things would be much better if I could get rid of the big old steel supply & return pipes that run from our hot water boiler (less than 5 years old) to the home's original cast iron radiators in each of the rooms on its first & second story. A set of large (2-3") pipes run just under the bottom of the basement ceiling joists directly above the landing at the bottom of the stairway that goes to the exterior door on the side of our house & then into the kitchen on the first story. I'm 6ft tall & need to duck quite a bit to avoid hitting my head on these pipes whenever I pass under them. In addition, they are preventing us from having a code-conforming bedroom in our finished basement because MN's 2020 Residential Code for Existing Basements states "the finished ceiling height shall be a minimum 6'4", including beams, girders, ducts, or other obstructions" & "a minimum 6'4" headroom is required in all parts of the stairway." My father was an electrical & refrigeration contractor & I spent 6 summers working for his company while attending high school & college, so I'm pretty knowledgeable when it comes to construction & mechanical "stuff." Unfortunately my dad's business was located in a small farm community where most homes didn't have natural gas, so I've got little to no experience with hot water & steam boilers/heating systems. I would like to replace the steel pipes going into the 1-1/4" inlets & outlets in each cast iron radiator with much smaller oxygen-barrier PEX pipes that can run through holes drilled in the ceiling joists. I've been reading every article/blog post I can find for this subject but unfortunately haven't found any that can provide me with the guidance I need to do so. I'd like to install two manifolds in the mechanical room where our boiler is located & then run two (supply & return) PEX pipes to each radiator in the house. From what I've read, it sounds like 1/2" PEX should be sufficient. So, do I need to run the PEX pipes all the way to inlet & outlet hubs in each radiator so I can connect them with 1-1/4" to 1/2" reducing bushings/valves or can I connect the PEX pipes to the existing steel pipes between the open ceiling joists in the basement so I don't need to try "fishing" the PEX thru the exterior & interior walls where the original steel pipes were installed when the home was built? The house's footprint is less than 700 sq ft so the pipes running to each radiator are not very long. There are only four radiators on the first story (front entrance, living & dining rooms & kitchen) & four more on the second story (3 bedrooms & one bathroom). One more thing - the hot water boiler was the house's only source of heat when we bought it, but we installed a Hartman pellet stove in the living room on the first story last winter that can heat the entire house on all but the coldest MN winter days & will be adding a 2.5 to 3 ton mini-split heat pump system with 3 high-wall units located in two of the upstairs bedrooms & the living or dining room this summer. So we'll soon have three different heating systems & will use whichever is the most efficient to operate at any given time. I expect we'll use the mini-split wall units to take the chill off in the early fall & late spring so we won't need to start the boiler or the pellet stove until the temps get & stay much colder. I removed all three of the cast iron radiators in the basement & expect the combination of the heat from the natural gas boiler, hot water heater & clothes dryer to keep it plenty warm down there, as I've had the exterior walls & rim joist cavities sprayed with 2-3" of high-density expanding foam insulation & all 3 of the windows are new high-efficiency ones. If not I can always add another min-split high wall unit down there. Any advice you have to share would be greatly appreciated! Thanks in advance for any support you can provide me with that will allow me to wrap this project up before my fall deer hunting season starts in September!