Indeed, if we look at Church's paper, we'd find that the term is 80 symbols written fully, {λt[{{{{{t}(t)}(λh[λf[λn[{{{n}(h)}(f)}(n)]]])}(t)}(t)}(t)]}(λf[λx[{f}({f}(x))]]), or 44 symbols when abbreviated, {λt t(t, (λh λf λn n(h, f, n)), t, t, t)}(λf λx f(f(x))), which aren't too impressive given the alphabet of 11 or 12 symbols.
I made the simplest choices I could that do not waste bits.
> And why use a self-delimiting format in the first place?
Because a lambda term description has many different parts that you need to be able to separate from each other.
> And why encode de Bruijn indices in unary
I tried to answer that in more detail in this previous discussion [1].
[1] >>37584869