However the universe is finite in this regard as you reach the frontier expansion wave of the BB there is presumably true nothing beyond and that that nothingness goes on indefintly.
You are falling into your own trap
How can "nothing" go on indefinitely? The problem is "nothing" is the wrong word, as it is too easy to confuse (as above) with "emptyness". More appropriate terms might be "non-existence" or "lack-of-existence".
Fortunately we do not have to deal with "non-existence" very much. There is no "frontier expansion wave" of the Universe. The Universe is either infinite or compact, which means in either case there is no edge. To say the Universe is expanding is to simply say that the distances between things (sufficiently separated things like super-clusters) are getting bigger. I guess our 4d Universe could have an edge where it adjoins some higher dimensional bulk space, which is a string/M-theory type concept, but even then it is not "nothing" beyond the edge - just the larger "true" Universe.
The Universe exists. That which is not the Universe does not exist. If it did, it would be included in with the Universe. Simple really
Actually, it can be more complex than this but that's for another day...
Whew, blows, my, mind
Good