For IG, it's much better to post it manually in the app itself in your phone (partner it with a good proxy if you're targeting a different country) since you can add trending sounds to reels or even in carousel, which you cannot do in third-party schedulers like Later. For other platforms, like Facebook, it's okay to use a scheduler.
What's your oppinion on buying massive amounts of aged instagram accounts, warm them up for 2 weeks and then post 3x daily via antidetect & proxy
Warming them up for 2 weeks or longer is the safest way to warm these accounts. 3x daily for each account seems too much unless you manage a team of SMMs/VAs, or if you have an automation to schedule that content on each account which I do not recommend. What we're doing is 3-4 quality content per week, share IG stories (this really boost the reach and engagement), and engage with fans.