Find the right Google OR-Tools algorithm
It will fit in the CP-SAT solver. You will need to scale floating point coefficients to integers.
The objective function accepts floating point coefficients. But (x1 + A1)^2 will propagate better if you keep it that way instead of A1^2 + 2 * A1 * x1 + x1^2. which fits into the linear with double coefficient limitation of CP-SAT, provided you use temporary variables sx1 = x1 * x1.
Then make sure to use at least 8 workers for that. (parameters num_search_workers:8).
Now, I believe there are least square solvers that are more suited for this.