You are not the only one to be underwhelmed by Spirecraft shield bearers. For a "one time damage absorbers", their current form has two big shortcomings.
One, its HP seems too low. Sure, 14 million HP may seem like alot, but when you consider it is supposed to take all the fire that would normally be spread across a fleet, along with the inability to repair them, 14 million does not last very long at all.
Two, once they get to low HP, their force-field size shrinks to near uselessness. For normal, repairable force-fields, this shrinking can be a good thing, as it hopefully allows other force-fields to start taking the "punishment", buying time for the weakened one to be repaired. Of course, since Spirecraft shield bearers cannot be repaired, this is nothing but bad news.
To combat point one, that should be easy, just bump up the HP even more.
To combat point two, I propose that there should be a "minimum force-field radius", that instead of the 1 or 0 radius that a normal force-field has at HP 1, the Spirecraft shield bearer would still have a decent sized force-field radius at HP 1. Naturally, that relation of HP to force-field size should still be linear, which would cause a Spirecrat shield bearer force-field radius to decrease slower.