Welcome ClearProp,
Many of the civil aircraft Test Pilots I know were drawn into the process because they already worked for the company in a different capacity. Their piloting skills were appreciated by the Flight Test department, and they moved up. I commonly get the reaction that they did not ever expect to become a Test Pilot, it just sort of happened, because they were int he right place, at the right time, with the right experience.
There would be varied advice and experience from others, but from me, I suggest that you get the non TP job with the company first, and form a good working relationship with them - the trust with the work and the aircraft. From that, your willingness to self fund more training should fit right in nicely, and you might advance.
A can say that the Test Pilots I know are all old guys, who are mellow and content to wait for the next opportunity to fly, rather than to "push the envelope" to fly this time. Advancing years, coupled with enough flying experience to have overcome the bling of it seem to be assets....