Plumbing.....I just bought a foreclosed home and had to replace the plumbing lines.?

I had to relocate the meter to another room so where the 3/4" line comes into the house, I increased the line to 1" and ran the line to the meter which I also upgraded to a 1" meter. After the meter I reduced the line back down to 3/4". So I installed a tee so I could feed into a new Navien Tankless Water Heater and then continued to run the 3/4" for both the cold and hot water run down the full length of the ranch style home I bought. My run starts with two drops, one for the laundry tub and the other for the washing machine.....then the run continues to the kitchen and drops for the kitchen faucet and continues through to the first bathroom where I installed a tee to supply water to the ice maker and ends to the tub in the bathroom. At the other end of that same tee in continues through to feed the bathroom faucet and the toilet. Then it continues through the other wall to the master bathroom and feeds another faucet, shower and toilet.....then continues to one last run to feed the upstairs bathroom faucet, toilet and shower. At the end of this run I have a recirculation return on the hot side that goes back to the tankless system that has a internal circulation pump. All my drop feeds are from 3/4" to 1/2" as well as the recirculation line is 1/2". With everything being connected, all the air is bleed out of the lines I noticed two issues.....first is that if two showers are in use, if anything else is turned on the the water pressure drops quite a bit. The second problem that I found was when my wife was showering and I flushed the toilet she got burnt cause all the cold water got directed to the toilet. Is there something I can do to fix these issue.....(ADDITIONAL INFO) I would also like to add that I plumbed the entire house with pex plumbing.....I love pex!.....so I was wonder if a expansion tank might help or something. With just one faucet on I haven't noticed surges in the water pressure. The pressure coming into the house was between 65 and 70 psi. I think I gave all the information I can think of.....thanks for any help you can offer me.