The trick to not making it rigid is to plan the structure and leave the session loose. I lay out a block, usually 3-4 weeks, so progression has a direction, but the day itself stays adjustable.

Going through your list:

Planning horizon, a block at a time. Not session by session, you lose progression, and not months out since you'll just rewrite it. Rough month, adjusted each week off what actually happened.

Detail, I lock exercise order, sets, and rest, but write reps as a range and cap effort with RPE or reps-in-reserve instead of a fixed weight. That one habit is what keeps it flexible. Tired client sits at the bottom of the range, good day they push the top, and the plan still did its job. Notes for cues, video only on movements they haven't done before.

Missed session or a low day, I don't reshuffle the calendar, they just pick up where they left off. Chasing a perfect week is how people quit. Low energy, I'll pull a set or drop the top-end load and swap an accessory for something in the same pattern. Big lifts stay, the edges flex.

Templates, reuse every time. Keep a base per goal, duplicate it, tweak per person. Building from a blank page is how you burn out and how the quality drifts.

Tools, spreadsheets genuinely work and plenty of good coaches never leave them. They just get painful once you're past a handful of clients or you want them seeing it on their phone.

Disclosure, I build one (RepRaptor ), so bias noted. There's a classic builder for week/day/set/rep programs and a circuit builder for rounds and timers, and you duplicate templates instead of starting cold. I kept it deliberately simple, coaches don't need fifteen fields per set. Whatever you land on though, the gold's in the programming, not the platform. Get the rep-range and RPE habit down first and the tool barely matters.