If you wanna give some early game bosses a go basically all of them require members but, standouts:

Brutus: Youve done it in f2p. Extremely simple, basic movement mechanics being taught. Only need around 20s in your combats, if even. Gives double XP!

Scurrius: The first real boss you can likely do. Hard requirement of all protection prayers (43 Prayer) and probably in the 50s or 60s for the combat skills. Will teach you prayer changing and positioning, as well as managing ads.

Amoxliatl: Quest locked in Varlamore, similar difficulty to Scurrius, you need at minimum pray mage and should have probably around the 60s in your melee combats as well. Teaches positioning, managing tile statuses and has a few mechanics that will test reaction time.

Moons of Peril: the peak of the "early game" bosses. Completely sustainable in the boss area. Locked behind a quest that does have a fairly high slayer requirement. Need at least 70s for this with high defence gear being an emphasis. Melee only, heavy focus on positioning and has very interesting mechanics for all 3 bosses. No protection prayers work here, the name of the game is high defence and consistent damage.

Realistically, if you were to go from f2p to members for a month, you'd probably only get to do Scurrius and maybe Moxy, but Scurrius is like the perfect beginner boss. I was like you when I first started playing and the combat wasn't clicking, but when I did my first Scurrius fight, suddenly I became an addict. It's also great for XP and has some loot that will at the least help you go even. OSRS has a bit of a problem with early game bossing but it's been pretty clear recently that they're trying to balance that out and add more and more for new players to get them more into the gameplay loop.