Technically, he still remembers how to use breathing techniques, but he doesn't have the physical capability to use them
Realizing the situation he was in, he began to stroke his chin as he contemplated deeply, focusing on how to tackle the problem. "Yoriichi's template also just ran out of duration, so I can't rely on breathing techniques anymore to fight back. Hmm... What to do, what to do."
Anime & Comics · Supposal
Yes, MC was using old Yoriichi's template to teach Teppei.
The routine continued, disregarding everything else as if they were contained in their own world. An arm that seemed to be on the verge of breaking, miraculously kept on to sunrise, performing a series of 12 moves in succession without break. Time flowed quickly before they knew it.
Anime & Comics · Supposal
Gathering an army of hilichurl is faster since he can snowball with numbers, and it only makes sense given that that's the strength of curse manipulation. Besides, his physical condition changes based on his template, so any physical improvement he gets as being "Geto" wouldn't carry over to a future template.
Realizing the situation he was in, he began to stroke his chin as he contemplated deeply, focusing on how to tackle the problem. "Yoriichi's template also just ran out of duration, so I can't rely on breathing techniques anymore to fight back. Hmm... What to do, what to do."
That Time I Got Reincarnated in Genshin Impact
Anime & Comics · Supposal