When the game was first released, the level cap was 50 and the maximum gear level was Mk 10 (I think), whereas now it goes up to level 65 and MK 15. Despite that, quite a lot of the story content was designed for either the original power level or one of the interim bumps, so it's laughably easy by modern standards. Value Subtracted is correct that there's a significant sudden bump in difficulty when you get to some Voyager-related content, but it's more in terms of needing to start to understand how to build a ship properly, not so much in having fancy/expensive gear.
And that's the real bottom line: if you know what you're doing, you can complete endgame content literally in a shuttlecraft. The DPS disparity between somebody who understands how the components of a build work together and how to fly their ship properly (from reading the wiki, DPS league build guides, r/stobuilds [unfortunately], etc.) and somebody who doesn't can be literally an order of magnitude or more, even with both using only "free" mission-reward gear and the same tier ship. I'm talking about 10,000 DPS for a typical random player vs. 100,000 DPS for a reasonably-competent one, and that's not even including the top-tier builds with expensive gear, which are pushing more like 250-500K.
If you want to easily complete story content without all that learning, the TL;DR is to get a cruiser, fill it with beam arrays of one energy type, fill your console slots with consoles that do +DMG for that energy type, and spam the Beam: Fire At Will ability as often as you can. (Even for a "simple" beam build like that, there's a lot more complexity in terms of skill tree, duty officers, traits, cooldown strategy, etc. that you could get into before even considering gear.)
Understandable. Honestly, if I had been in a position to make your choice I probably would've gone for the Temporal Light Cruiser too. (I mean, the Atlas maybe has more deeps, but the Connie's a Connie....)
I forgot to mention it before, but one of my other characters is a TOS-styled Klingon (I wish that was a real separate subfaction, but that's a different discussion) who flies a D7 Temporal Battlecruiser. It doesn't "feel" as powerful as my other character's Atlas, but that could be down to the fact that the Klingon is a newer/lower-priority alt with less optimization and/or other differences in the build, not necessarily the differences between the ships themselves.
Speaking of which...
(Also, if anyone wants to comment on how good their equivalents are, I would be happy to replace a Constitution-class with a (captured) D7 or T’Liss.)
The D7 is IMO a straight upgrade from the Connie with the addition of the cloak, although I guess YMMV depending on how you feel about replacing the universal/command seat with universal/intelligence. However, it's not a Connie, so I'd say skip it for space barbie reasons, unless used on a KDF character after the TOS FED one has already been squared away.
The T'Liss is even more mechanically interesting, since it's got singularity core abilities (but no cruiser commands), an experimental weapon (while keeping the 5 forward-facing slots!), and adjusts the seating to have cmdr tactical/temporal instead of cmdr engineering/temporal (which is a double-edged sword because you gain the possibility of CSV/CRF III, but are stuck with 3 eng-only slots where you could've loaded up with temporal abilities before) and universal/pilot. That's all pretty neat, but again it's still not a Connie, so I'd say save it for a Romulan alt later.
Mirror universe Alexander would probably still have two living parents.