glad to see an interest in the tier! your assumptions were pretty much all correct, though we do have a viability list.
decid also functions as a potent and at times oppressive sweeper with its wide attacking options--stab cc, triple arrows, and leaf blade, and coverage options like knock, sucker, and u-turn--bolstered by sd. zard and typh were staples on sun, which was indeed affected by nu's ban on drought, but zard is still serviceable as a standalone attacker thanks to its useful speed tier and high special attack. it also sees play as a physical sweeper with sd, dd, and belly drum, and moves like flare blitz, eq, thunder punch, and acro. typhlosion is a bit more one-dimensional as its coverage is way more lackluster than zard's, though it boasts identical stats and a powerful stab eruption.
samurott has seen better days. it was once a scary attacker that could go physical, special, or mixed, with move options like sd, knock, and flip turn, and held items like boots, lorb, and mystic water. it was a wild card that could mow down teams if you didnt know its set. it's mostly fallen out of favor because of faster and/or stronger water-types, and often competes for your team's water-type slot with qwilfish, which provides invaluable utility. and meganium is meganium. it's got some cool new tricks like knock off and dice + bullet seed, but offensive sd sets kind of have 4mss (you want sd, stab, eq, knock, synth, maybe even tera blast rock) and just fall short in the role with other grass-type mons like the aforementioned decidueye, or ones with more immediate power like scovillain and even alolan duggy, or the insane speed of hisuian electrode, or heck even dipplin is better
unranked? Great Atk and Speed tier, good ability if you are able to predict Volt Switch from the new Rotom-Mow for example and great coverage options in my opinion (being even possible to use it as an special attakcer)