doing this: https://openwrt.org/docs/guide-user/additional-software/imagebuilder
by using this: https://openwrt.org/docs/guide-developer/imagebuilder_frontends
I'd remove ppp ppp-mod-pppoe, unless you know you need them, and add the packages you listed above.
Hopefully you won't break the max size barrier.
Another option it to downgrade to 19.07, it'll be smaller, and leave you more space for additional
packages.
What's the use exact case anyway, you might be able to strip it further.