Just an additional point of clarification, since you're new to the traditional game. Backyard practice on the same target over and over is good from a repetition standpoint and getting your form to be solid and the shot process to be second nature, so this should not be discounted. However, as you get close to getting in the woods, you want to be challenging your mind with practice at different distances and not seeing the same 'sight picture' as when you walk out to your back yard. A buck in the rut walking in on you from a different direction than you expected in your tree stand will challenge your 'normal shot' routine. Be ready for that by getting out and shooting 'quickly', but still with good form on a 3-d target. Your squirrel hunting idea is good too. Nothing like live critters to challenge your shot sequence.
John Schultz in his video quoted Howard Hill used to saying, 'if your form is good, you won't be to far off at 60 yards......'. For us mere mortals, translate that to 20 yards.
If you're shooting tight groups in the back yard, your form is probably pretty consistent.