If you planning to buy Phone (1) now, I’d suggest you to go with Phone (3a) which essentially comes down to the same price but offers a better SOC, camera, screen than Phone (1). Also, I think Nothing OS 3 is the last major update for Phone (1) and it might receive security patches for 1 year now. But if you take Phone (3a) you would solidly be able to clutch it for at least 4 years with major updates and regular security patches. I would not even suggest Phone (3a Pro) since the price bump does not justify the feature that it offers than Phone (3a)
Also, on a side note if you are looking for a true flagship like experience and budget is not a constraint, I’d suggest you to wait for Phone (3). Hopefully that would be a major upgrade on the flagship lineup