The problem being that we only have 26 letters to represent 40+ sounds so a combination of letters are used to represent some sounds.
That is not the problem at all.
Combining several letters to spell a sound, whether (ch, igh, e-e) would not cause any problems (chat, high, even), if they were used consistently. English reading and spelling difficulties are caused by the irregular and unpredictable use of them.
Learning to read English takes vastly longer than the European average of one year (with just three months for Finnish), because 69 letters and letter strings have more than one pronunciation (fat, man – many, father, able... shout, out, loud – should, shoulder, soup, double... even - ever... define - engine, machine).
Learning to spell English takes an average of 10 years because nearly all 44 sounds have more than one unpredictable spelling and at least 4,000 common words require memorisation of variant spellings (bed, led - said, head, friend, any...speak - seek, shriek... hair, care, their, there, bear....).
The problem is unpredictability.