While the groundhogs and their mounts are a creditable threat, My bigger concern is the squirrels. Not only are they fast, they are unpredictable, hence the term, "squirrelly".
And they have a track record from the Robo Squirrel going after that one cereal, to the ones that help with your heartburn, but steal your peanuts....
Raccoons, now, they will only steal your van, and get pizza!
.22 Rimfire, unless you make up super light loads for your .222.
Your 7mm is a good choice, personally I prefer a .30 but that's jut me.
The .45-70 in a strong modern gun like the Marlin is tough to beat, except for long range shots. You can shoot a .45-70 quite accurately at long ranges, but its something that takes more skill than most people are able to take the time to develop these days.
If you think you need something with more punch at longer ranges than what you currently have, take a look at the .375H&H. Its not just for Africa. With 235 & 270 gr bullets it has essentially the same trajectory as the .30-06 with 150 & 180gr bullets, but hits ...a bit harder. Not cheap, not for everyone but if you have a need (or just a desire) it gets things done!
your collection covers everthing pretty well already. You might consider a round in between the .243 and the 7mm Mag. Something with a heavier bullet than the .243, and less recoil than the 7mag. The 6.5x55mm Swede is an awesome performer, flat shooting, low recoil, and 140/160gr bullets penetrate well. The modern equal is the .260 Remington.