You know, I find it interesting people keep bringing these up...
Bards aren't in every party. In 4 years now, I played a bard (getting just to 5th level in that class) once and no one else in my group (or the new group I'm in) has played a bard.
Guidance is available only to Clerics and Druids. While most parties tend to have one or the other (if not both), the spamming use of guidance has made a lot of people nerf the use or eliminate the spell altogether. In my own games, for example, guidance is only usable once per short or long rest for each PC.
Expertise is available only to Rogue and Bards. Xanathar's brought in Prodigy, but to limited races, and it wasn't until recently with Tasha's that you have it readily available if that is what you want, but competing with so many other feats it is by no means a guarantee. Of course, many classes have features which double proficiency bonus to a limited subset of skills, but those are subclass specific and limited as well.
While many people here seem to feel any or all of these things are a given, please know they simply are NOT universally so.
All that being said, what is nearly impossible should not be exceedingly rare IMO for tier 4 PCs! It is a huge let down when the situation does arise, the PC has a chance to do something great and memorable, but the odds are so against it that the PC might as well try something else.
So, for groups where you don't have power-gamers, min-maxers, or tactical-team experts, most modifiers to ability checks range from +3 to +10 with the only common exception IMO is the rogue due to expertise sometimes getting in the +13-16 range.
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Perhaps another route, which would be simpler than adjusting DCs and tasks names, is just to allow every creature to have expertise in one skill.
For PCs, perhaps limiting this to their Background skills would work best. Then, by the time they reach tier 4, they will probably have +14-17 in that one skill they can excel in.