Any of these would work:
1) Give it Vampirism but allow it to target only fleet ships. Possibly fiddle with the reload and salvo size a little; it needs to kill slow enough that its own attacks can't keep repairing your losses alone. Now you've got an alternative to repairing it, all you need to do is keep it alive. This is assuming Vampirism only gives you at most the health of the destroyed victim. There is no way it should be healing 600k per Fighter I it kills.
2) Reduce its range to 3k, give it x10 damage to Mark I-IV Guard Posts and Vampirism (but only against Guard Posts). Effectively it can convert Guard Post health into its own. Maybe give it a 2:1 on health healed. If it could heal around 8-10% of its health per Guard Post, that would give you a better method of repairing it. Of course, Guard Posts never respawn, so they are a limited resource to feed your Regen Golem and you need to get pretty close so you'll probably take some damage getting in there in the first place.
3) Make it a mobile space dock like the Neinzul Enclave, except it spends its own health, and it ignores build time. So if you click on Fighter I, it instantly spawns as many Fighter I's as it can (up to your ship cap) at the cost of its own health. Only works while it has at least 10% health. For extra fun, let it function as a Adv. Factory to make Mark IVs.