Usually the game makes you complete 4 cycles to get platinum but using this guide you can get it within 2 runs.
You should only go for this if you don't want to fight bosses repetitively and don't mind absurd EXP grind.

Note: This is not an in-depth guide on how to obtain individual achievements as there are countless guides available for it, instead focus of this would be to obtain a few specific achievements which makes you finish the game 4 times,
- Man Without Equal
- Master of the Prosthetic
- Different endings

For your first playthrough,
- Aim for Purification ending in your first cycle. (You will have to defeat 11 major bosses for it)
- You also have to defeat following optional bosses: Headless ape & Demon of hatred.
- Grab all the six lapis lazuli (2 dropped from Demon of hatred, 2 can be bought from the fountainhead pot merchant, 1 dropped by fountainhead shichimen warrior, 1 obtained after completing Great carp quest)
- Once you are done with purification ending try and clear all the miscellaneous achievements that you can before starting NG+, achievements you can't have by the end of your first run are,
1. Man Without Equal
2. Master of the Prosthetic
3. Remaining 3 endings (Dragon return/Immortal severance/Shura)
4. Height of Technique (Possible but tedious, best to tackle in the next cycle)
(Apart from these^ try and clear everything else)

Once you are done with above start NG+
- Make your way to the point where you make choice with Owl i.e after obtaining 3 key items Lotus of the palace, Mortal blade and Shelter stone. (Make a backup of your save before talking to Owl)
- Go for Shura ending by selecting Obey the iron code and defeat Emma & Isshin Ashina. (This should trigger Man Without Equal and Shura ending Achievement)
- Restore your backup and this time choose Break the iron code, defeat Owl and make your way to the fountainhead palace. (DON'T DEFEAT DRAGON YET)
- Do the prerequisites for Dragon return ending and obtain Frozen tears.
- Grab remaining 4 Lapis lazuli (2 from pot merchant, 1 from shichimen, 1 from carp quest)
- Once you have frozen tears go ahead and defeat Dragon & Sword Saint Isshin (Make sure to create a backup of your save after taking final deathblow on Isshin, it's important that you don't choose an ending before making a backup)
- Get the dragon return ending > Restore your save > Get immortal severance ending
Ideally you should have all 4 endings and their achievements by this point, the only ones you are missing would be Master of prosthetics & Height of Technique
It's time to farm for those skills points and prosthetic upgrade materials which is a long process but possible in the current world state.
We already have 10 required lapis so once you have farmed enough for remaining upgrade materials you should be able to get your platinum in your current cycle itself.
If you have any question regarding Sekiro's platinum feel free to drop it in the comment section.