@wera44: The frame-up of your question resonates with me as I both live in Georgia and have an auto racing background. I completely agree with you that purchasing a ready to go racecar is FAR less expensive than building your own, however, there are many facets to any sport and if you are a person who enjoys the research, hunt, acquisition, testing and experience of building then, there is nothing wrong with that.
Like racing, an important early decision is: in what class am I going to race in. While your goals may change over time, it would be good to be right about what you are going to do with your new rifle. Is it for long range only or will it also serve as a hunting rifle and, what is the nominal distance you will likely shoot and what is the longest distance you will likely shoot. This is like setup on the racecar in that you have to decide which corner or corners to set the suspension up for.
There is a lot of good advice above about custom actions, barrels and stocks but let me relate a few things:
The Bergara action is fully Remington 700 compatible in every way (triggers, scope base and stock inlet). The HMR and HMR Pro are both very good target rifles with great (adjustable, pillar bedded and inner mini chassis frame) stocks and accurate barrels. Any gunsmith who can make a Remington 700 barrel and make a replacement barrel for the Bergara. This is a good option and one I have recommended to several folks starting out with longer range shooting. If you know you want to compete in the future, this is a great trainer to get started and something you can keep and upgrade for years to come. That said, there are other options if you know competition is in your future.
Remington 700s are far more accurate than most people give credit. As it turns out, the magic of accuracy comes from the the barrel first and then the bullet. After that comes a lot of pre-work to the brass, stock, scope base, rings & etc. That is like car race prep. The Remington 700 is the Ford Mustang of car world. There are limitless options for every single part of that action and some of the work to make it better can be done by the owner. Additionally, you can purchase Rem/Age barrels for a 700 now that allow you to swap barrels at home with minimal tools and they shoot pretty darn well.
Custom actions can be a good shortcut to improved accuracy and many renowned gunsmiths will chamber and thread barrels for numerous actions based on specifications alone. As a lathe owner and [very] amateur 'smith, I like to have the action in hand, especially for competition rifles, to set the headspace exactly where I want it and for other reasons of timing the barrel. Buying a custom action really commits you to an expensive finished product. At a high level you'll buy a $1000+ action, $600 barrel, $500 stock, $250 trigger, $200 bottom metal, $100 scope base, $40 recoil lug and, possibly, a new scope and rings. These are round numbers and the costs could vary wildly if you find sales, purchase used or substitute parts for less expensive ones but it is a reasonable guide.
Tikka makes a very good action but to swap barrels you'll want to cut a more pronounced shoulder on the action. This is no big deal but there are very few triggers for Tikkas and the barrel threads are metric. Again, not a big deal for a competent smith but different from most.
I have found that Ruger RPRs vary in consistency. After watching a guy at my range shoot a few .4 MOA groups with factory 140 grain ammo in his 6.5 Creedmoor RPR, I purchased one that would shoot no better than .75 with the same ammo. This was not a shooter issue
While there is good aftermarket support for this platform, I'd prefer to have a Remington 700, 700 clone or custom action to build from.
Without really knowing what your goals are, I would recommend an HMR or a Remington 700 unless you know you want to compete. I would have a different recommendation for that depending on what you are interested in. The reason for this recommendation is the universe of aftermarket parts that can really be put together to do anything you want it to do.
I have tried to stay away from the caliber discussion but it is worth mentioning. The 6.5 calibers are great as they and offer reduced recoil as compared to .308 and reasonably long barrel life as long as the speed is reasonable. 2700 FPS for 140 grain class bullets should give you 2500-3000+ rounds of very accurate barrel life. 6mm calibers offer even less recoil but in order to realize their advantage you have to shoot at speeds starting at 3000 FPS. This will diminish barrel life and in my experience getting to 2000 rounds while maintaining good accuracy is difficult.
Lastly, the current ammo and component shortage is pretty daunting. If this situation really is just the result of significantly increased firearms sales driving significant purchases of factory ammo AND hoarding by the reloading community then we should see some relief by the end of Q1 2021. If there is something more nefarious going on, buying a new firearm now might be silly. I wouldn't buy anything right now unless you get a screamin' deal.
Henryrifle