If you are going for the ability to easily swap between cartridges as you mentioned, my suggestions would be to:
1. Go with a custom action that you can buy pre fit barrels for as previously mentioned. Or...
2. Go with a Remington 700 (since you mentioned you want to stick with a factory action) and go with a Remage barrel setup. Or...
3. Get a Savage. There is some hate out there for Savage but I have not personally no had ANY of the issues that people love to hate them for. Truth is you can make them into damn fine rifles in my experience. I have one that runs really well.
A custom action would be my vote but the $ involved in dedicating yourself to a full custom build, while worth it, can be a lot up front. It would give you the ability to buy all the pre fits you want. If you want really easy barrel swaps, got with something like the Terminus Zues QC option. An Allen wrench is the only tool needed and you can change your barrel in 60 seconds.
If you are firm on the factory action options, the Remage setup is a hybrid design between Remington and Savage hence Remage... It allows you to use the barrel nut design from a Savage on your Remington action. Both a Remage setup and a Savage will allow you to set your own headspace which means you would not need a gunsmith for your barrel swaps. Keep in min that these options can end up leading you down the $ rabbit hole over time and before too long, you end up realizing that you spent damn near the same money as if you had just gone with a custom action from the get go.
Make sure no matter what route you go, you get a set of Go/No Go gauges for each cartridge you are working with so you can properly set your headspace. Some may suggest you only need a Go gauge and can use scotch tape to make a No Go gauge... Up to you but I prefer to have the proper tools.
Also... Keep in mind that if you are running all these different cartridges, you may end up needing more than one action. 6.5 Creedmoor is a short action cartridge but a 300 PRC is not...