The “seconds” are really just a way of breaking open what happens inside a 6-second round, not a claim that time is literally being tracked continuously. In practice it probably is closer to an action-point system, just with a fiction-first label that matches the six-second round the game already talks about.
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So I’m less trying to model real time passing and more trying to make tradeoffs between attacking, casting, and positioning clearer and more flexible than Action/Bonus Action currently allows.
My personal preference is to use ingame-references (like seconds) instead of meta currencies (AP, Action/Bonus Action/Reaction), whenever I can get away with it (that's why I also hate the Bastion system with its Bastion Turns - dude, just use weeks!).
My perspective is that you are attaching a fiction-first label to a meta currency. So long as characters are still taking their actions and then (mostly) freezing in place and letting the next person act, and so long as combat rounds aren't then taking 6 second per combatant, then this isn't fiction-first. It's renaming action points as seconds, but it isn't actually making them into seconds.
If you reworked the system and made a simultaneous-action system where everyone did have 6 seconds worth of actions (each with a specified duration) during the same 6-second round, that would be fiction-first. That is much more of a re-write and I understand why you don't want to.
Anyways, as it stands, it's a perfectly fine slightly-more-fiddly action-economy system. There's some combination of actions this allows that standard 5e doesn't that leads to some bizarre results, but welcome to rules tweaks. Mostly I imagine it will be a lot of re-writing and re-learning for minimal change in behavior.
I want to be clear that this system has not been playtested yet. I am posting it here specifically to get feedback before running it at the table. EN-World has a lot of people who have thought deeply about action economies, martial and caster balance, and alternative systems such as Pathfinder Second Edition or GURPS. I am very interested in where people think this would break in practice, what edge cases I may have missed, and which classes or play styles might need adjustment once it sees real play.
A dominant takeaway I have from GURPS (3e, where it matters) is that it is decidedly not built to make different choices be equal or balanced. There are strictly right choices. Other times, there are not, and those are often because of natural consequences of the decisions. It's helpful to understand those in deciding what GURPS options to implement (or how to apply this to a proposed alternate system, like you are doing here). One good example is 'taking a turn to aim/recover' and the implications are different across tasks/genres.
In GURPS, you can swing a sword in 1 second, or swing an axe. Both do cutting damage based of your Str-based swing damage dice, so they are even there. The axe adds more damage to that score, so does overall more damage. However, you have to spend a second to recover and can't attack again (and IIRC can't parry) until you do*. With Damage Reduction and stun thresholds and overall just lower HP compared to something like D&D, it is conceivable that the extra damage on round 1 (and maybe never experiencing the counterattack) justifies the lack of attack (and having to dodge/block instead of parry) on round 2. Possible, but highly unlikely, and I don't think I ever saw anyone use an axe in the game for very long.
*This is relatively realistic (why they likely did it), but they leave out some potential realistic other mechanics that advantage axes over swords other than a little extra damage.
Conversely, missile weapons work better when you spend a second beforehand to aim. There are various rules about penalties for 'snap shots' as well as bonuses (or removal of range penalties) for use of scopes and the like that require aiming. You can just shoot every second* (depending on the loading time of your weapon) and accept any penalty that entails. However the penalties can easily cut your chance of hitting by more than 50%, making shooting every other turn a perfectly reasonable decision (potentially relegating shooting during the first turn to 'cause I won't get a shot off if I wait til turn 2' situations).
*or more than once per second, with rules for automatic fire or fanning a single-action revolver, and so on.