BBT is not "demonstrable" even all the way out to the CMBR.
That's not in dispute since, as I said, it is weak around z > 10. The CMBR is z = 1089, but the CMBR is itself demonstrable and has met the predictions of the BBT on many levels, including the isotropy, anisotropy, power spectrum, wavelength, etc. The CMBR was about to be discovered with microwave antennae by Peeples & Dicke (Princeton), but the Bell scientists stumbled into it as a noise problem. The Princeton team knew, as did others (e.g. Gamow), that the Recombination event necessarily would be in the microwave band to reveal the event when expansion cooled the universe down to about 3000K.
Hawking once stated that it was the greatest discovery of all time, IIRC.
And the observations much closer (or later or however you want to interpret redshift and faintness) have some major problems as well, which requires "dark matter" that does what makes the model fit, and avoids doing anything that doesn't make the model fit the observations.
I too don't like anything that appears as
ad hoc science. The current model works very well, which does require dark matter. So was DM invented to accommodate BBT, or was the model made more robust after its discovery? DM was first discovered in 1933 (Fritz Zwicky), but there were not other lines of evidence to support Zwicky's hypothesis. Once the velocity distribution of stars orbiting Andromeda revealed far more matter had to be incorporated to explain their non-Keplerian motions, the DM began to get some real attention. I was fortunate on a McDonald tour to meet with an astronomer during his run to measure DM around dwarf galaxies in order to improve the distribution of DM models for various size galaxies.
Just because we can't measure DM directly doesn't negate its existence. Black holes also cannot be measured directly. Both of these, however, can be measured indirectly. It's fact.
But, being science hypotheses, perhaps something better than, say, MOND, will come along and make science better. But to suggest something is feeble that is tougher than iron is unfair.
DE, however, is conjecture. It is a problem because there only seems to be one way to measure it.... acceleration rate of the expansion. The error bars likely need to shrink in SN studies to better determine its strength, perhaps even its existence. But it is installed in the current model of BBT, so it has something going for it, though your suggestion that it may be a problem may not be invalid.
Beyond the CMBR, extrapolations down to a singularity, or just short of it at "Planck Space", are based on violating GRT with the conjecture of space "inflating" at amazing rates.
It's pointless to argue, IMO, any theory that has a t=0 beginning, IMO. BBT does not come close to addressing the pre-Planck time moments because science is likely never going to be able to do this. Of course, some scientists pretend otherwise because it has brings attention and notoriety, perhaps.
But, I am not seeing anybody address how time itself is not altered in calculating those rates, despite the indication from GRT that proximity to mass slows time rate. The whole effort of the BBT "fan boys" (to quote the OP) is to make the BBT fit, not to ask what those fitting efforts imply about things that would make it not fit.
Better still would be demonstrate how the "not fitting" would falsify the theory, and win the Nobel. The tons of efforts to tweak the BB model are rarely as ad hoc as you suggest. Any new discovery needs to see both how it fits BBT and how it doesn't. If it does fit the theory, then this is less valuable than if it doesn't. Anomalies will reveal more than another item on the Big Bang Bullet list, not that I would object.
I Once the BBT gets to the eras where it takes quantum level thinking to explain astronomical observations, I think the BBT is on very thin ice.
When in high school, we drug a John boat out to the middle of an iced lake in order to ice fish with some way to safely return if the ice broke. It was a little tiring to drag it out but more tiring to chop through the thick ice, which we never accomplished.
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It doesn't hurt to assume the ice is thin, but it is wise to consider all the given data for how robust it is.
"Good science" requires and open mind. And it also requires that the things passing in and out of minds have an observation-based filter that distinguishes between what is observable and what is conjecture.
Astronomers are well-aware of observational bias. BBT is not conjecture; this is a closed-minded viewpoint.