Isekai / Reincarnated TRASH [DO NOT READ!!]

Harem
https://www.novelupdates.com/series/disciple-of-immortal/

https://mangadex.org/title/8836eba0-a5de-4c21-b776-6f7740c1763d/the-disciple-of-lich

https://www.mangaupdates.com/series/8fa923x/the-disciple-of-lich

https://ncode.syosetu.com/n2494fq/

Harem
https://mangadex.org/title/61e5cfcb-aff4-4916-ac20-132f4542e451/sen-no-skill-o-motsu-otoko-isekai-de-shoukan-kemono-hajimemashita

https://www.mangaupdates.com/series/ru9kew5/sen-no-skill-wo-motsu-otoko

https://ncode.syosetu.com/n2626em/

Harem
https://www.mangaupdates.com/series/2fus2mx/gomi-ika-da-to-tsuihousareta-shounin-jitsu-wa-zensei-kensha-desu

Harem, Polygamy
https://www.novelupdates.com/series/the-unsuccessful-yet-academically-unparalleled-sage-a-cheating-s-rank-sorcerers-post-rebirth-adventurer-log/

https://www.mangaupdates.com/series/l3j820t/rakudai-kenja-no-gakuin-musou-nidome-no-tensei-s-rank-cheat-majutsushi-boukenroku

Reverse Isekai but Harem (all females have interest in him)
https://www.novelupdates.com/series/d-genesis/

https://www.mangaupdates.com/series/67gsbd5/d-genesis-dungeon-ga-dekite-3-nen

Ecchi
https://www.mangaupdates.com/series/88gruop/ansatsu-skill-de-isekai-saikyou-renkinjutsu-to-ansatsujutsu-o-kiwameta-ore-wa-sekai-o-kage-kara-shihai-suru

based on web novel

Harem (main premise of the plot to save both heroines so obviously harem)
https://www.novelupdates.com/series/flowers-in-both-arms-swords-in-both-hands-when-i-fell-seriously-in-love-with-the-heroines-of-a-mecha-game-my-whole-world-changed/

https://www.mangaupdates.com/series/58fa8ei/sekai-saikyou-no-kishi-wa-kanarazu-shinu-heroine-wo-sukuu-tame-isekai-demo-saikyou-no-kishi-to-naru-ryoutei-ni-hana-wo-ryoute-ni-ken-wo

Ecchi, Harem

https://www.novelupdates.com/series/isekai-cheat-survival-meshi-wn/

Harem

https://www.novelupdates.com/series/tough-another-world-reincarnation/

Harem
https://www.mangaupdates.com/series/pjp7nv7/ohitoyoshi-shokunin-no-burari-isekai-tabi