I think it's important to note that a seatcheck by a CPST is not so that the technician can install your seat, but for the technician to teach *you* how to do so under almost any circumstance

It does your child no good if you feel unprepared when you leave the seatcheck -- we are not providing a service, we are educators

Granted, it can be a real challenge to provide every parent with all the skills needed to address the issue of frequent travel during a quick event where a line of cars await seatchecks, but many areas do offer free private appointments where we have the freedom of more fully answering all such needs....
IME, the
Britax convertibles fit the vast majority of vehicles more easily than most other seats & without requiring any "tricks" or "tools". They are more expensive than most seats & they are indeed heavy, but I'm a single mom with a chronic illness & I've managed to tote my kid +
Britax through airports (all I'm saying is that it isn't impossible to do

)
For infants & small toddlers, the
Graco SafeSeat Step1 also fits most vehicles & airplanes comparatively easily.
Most combination seats tend to do well in these cases, too, even the Apex which is quite large.
Worse comes to worse, the Tote-N-Go is a last resort option for children over 12 months old. It crash tests well, but is very time consuming to fit the harness to the child & it pretty much never fits any vehicle according to the standard of less than 1" movement (because it doesn't have any base whatsoever).
Now that my daughter has outgrown every
convertible & combination seat on the market, we could use a
booster. We could detach & pack the highback of some
booster options & carry the base. Instead, though, we use the Ride Safer Travel Vest. It's more lightweight & spreads crash forces out over a broader area of the body. However, it provides no side impact protection. The RSTV + TNG are "niche" seats, not ideally suited to primary/daily use.