All basic tools: yes.

All upgrades to the tools: no.

All Bosses: No. But with backup + reload yes.

Now I count the Great Colored Carp and the Great Serpents as bosses, though they don't give a boss memory.

I don't count the Headless and the minibosses. Just do the minibosses as soon as they're available, if you want to fight them all. Some become unavailable later after the key events:

• collecting the Shelter Stone and the Lotus of the Palace
• defeating the Divine Dragon

So minimal time investment strategy to experience all bosses:

• Acquire the Young Lord's Bell Charm to unlock the first Hirata memory. Defeat the optional boss Lady Butterfly there. This is meant for early game. Can be procrastinated till endgame, but it's a hassle (the old woman who normally gives you the Young Lord's Bell Charm has died and a certain enemy drops it instead).
• Backup the save state before speaking to Owl at the top of Ashina Castle, the "Castle Tower Lookout" at dusk. Side with him, betraying Kuro. Defeat the bosses Emma, the Gentle Blade, and Isshin Ashina. This results in the Shura ending.
• Then load the backup but this time side with Kuro.
• This results in a fight with the boss Great Shinobi Owl. Acquire the Father's Bell Charm to unlock the second Hirata memory after this fight now (or earlier). This cannot be procrastinated, it becomes unavailable after defeating the Divine Dragon.
• Do the steps for the Return ending. There you fight the Great Serpents.
• Do the Great Colored Carp quest.
• Defeat the optional boss Demon of Hatred.
• Defeat Owl (Father) in the second Hirata Memory. This unlocks the option to choose the Purification ending.
• Defeat the final boss, Isshin, the Sword Saint.

You have now fought all the bosses.

You can now even get all remaining endings if you backup + reload the save after defeating Isshin, the Sword Saint, before speaking to Kuro at the Silvergrass Field for the last time.