Therband for your feet, definitely. Great for strength and flexibility.
As for your back, yoga is great for it. Stretching is of course great, but yoga incorporates strength, breath, and flexibility. Breath is super important while stretching. So doing poses like royal dancer, king pigeon, bridge, wheel, scorpion, camel, upward dog, and other deep backbends and holding them for about 5 breathes are really good for back flexibility.
Here’s my beck flexibility progress after one month of yoga:
If you think your ready or close to being ready, bring it up with your teacher. Ask them how much longer they think you need and if they have any tips. Let them know it’s a goal of yours!