Pixel 10a is essentially a 9th generation Pixel with the 9th generation Pixel SoC. It does have the greatly updated cellular radio introduced with 9th generation Pixels unlike the Pixel 9a so it will have much better battery life when using cellular.
10th gen Pixels other than the Pixel 10a moved to a new SoC not based on Exynos beyond the cellular radio and small parts of it. There are a massive number of differences far larger than the changes between any of the 6th through 9th generation devices. At the software level, the Pixel 6 shares more in common with a Pixel 9 than the Pixel 9 does with the Pixel 10 since the drivers are so much difference. The CPU cores evolved a lot over the years and are still ARM Cortex cores but nearly everything else other than the cellular radio is different for the Pixel 10. It isn't straightforward which is more secure. In the short term, 9th gen Pixels are more battle hardened and don't have as many bugs. In the long run, 10th gen Pixels will likely end up better off for security and robustness.
Titan M3 is coming with the Pixel 11, not the Pixel 10.
Our recommendation for security is getting an 8th gen or later Pixel. For performance and battery life, the non-10a Pixel 10 devices are best. For cameras, the Pixel 10 Pro and Pixel 10 Pro XL are best. Pixel 10 Pro Fold cameras are a downgrade from those due to limited space but it folds out into a tablet. Future folding devices will hopefully avoid the camera downgrade.