Excess XP is lost forever, as things currently stand. So if you want to
rebirth, it's best to do it as soon as you hit 675M, or not at all.
To do it or not to do it? That is ultimately up to you - a triple rebirth
gives you 3 extra levels to play with (thus opening some more builds), some
fairly powerful races, rebirth heal, and an option to join a different house -
these are decent bonuses, but none of them is really game-changing, especially
if you mainly play solo. Is this interesting enough to you to warrant losing
675M exp and whatever guild reputations you had? Only you can answer that.
As for a "playable" character, it really depends on what you consider playable

Obviously you're not going to hit level 100 anytime soon, but there are
viable solo builds under 50M (the tzarakk/riftwalker/ranger "holy trinity",
possibly more depending on what gear/etc you have).
Your secondary can use your ship if you give them the priv, same for gear (but
you will get unrepairable damage on the gear if it has weartime from both
chars).
What I did was start as a riftwalker, got all that sweet explore XP and level
quests, then made a merchant and did a lot of MIPs (burned about 45M cash on
that), ending up with about 100M totals to play with, which offers some decent
options already. Continued as tdruid after that. Obviously your options will
vary depending on resources available to you, but if you know the game and
have some gear, you can build up a character fairly fast.
If you are "on the fence" and can't decide - my recommendation would be to go
ahead and do it. Otherwise that "what if" thought is going to gnaw forever,
and you stand to lose more later on