Best approach for 'manual' common subexpression elimination

Question

I have working code, but am looking for ways to make it more elegant.

I have a fairly large expression with plenty of repetition and structure. A shortened toy example is

expr = (a/(a + b))^4.5  + (b/(a + b))^3.5 + 
  (a/(a + b))^1.5 + Cosh[a + b]  + 
  Log[a/(a + b)] + Log[b/(a + b)]

I have prepared a set of rules that simplifies the expression, for example like this:

rules = {
  a + b -> sum, 
  a/sum -> g,
  b/sum -> h}

In my actual code I use pattern matching, and also create unique symbols for all the matches, see below for an example.

 Derivative[l_List, i_][δ][xv, ρ] :> 
  Symbol[StringJoin["$δd", StringJoin[ToString /@ l], "x", ToString[i]]]

The simplified expression, and the actual set of replacements is provided by the following function:

replaceone[expr_, rule_] := Block[{newtemps, unique},
  unique = Union@Cases[expr, rule[[1]], Infinity];
  newtemps = Thread[(unique /. rule) -> unique];
  Sow[newtemps];
  expr /. rule
  ]
{newekspr, temps} = Reap[Fold[replaceone, expr, rules]]
temps = Association[temps]

I can then prepare a function for efficient evaluation of the expression, like this, with a nested Block.

exprfun[av_, bv_] :=
 Block[{a = av, b = bv},
  Block[Evaluate[Keys[temps]], KeyValueMap[Set, temps];
   newekspr]]    

I can Compile this function, although I suspect that the result is that the entire expression is expanded and simplified again.

Compile[{a, b}, Evaluate[exprfun[a, b]]]

Is this the best way of doing this?

Can I write a higher order function that takes my original expression, and a rules set, simplifies it and provides a optimised, and possibly compiled function?

What is the benefit of a nested With, and LetL, that I have seen elsewhere, compared with this approach?