ok then, let me explain:
in 5e, you're expected to be hitting enemies about 60% of the time. of course, many DMs up enemy AC so you'll be hitting less then that - but the game was designed around hitting about 60% of the time, meaning you hit on an 8 or higher. that's the intent. and the encounter rules say that one creature of a CR equal to the party is a good fight for a party of 4 players.
the CR table shows this pretty well - CR 1 creatures have 13 AC (13-8=5, 5-2 (your proficiency bonus at level 1) is 3, which is almost certainly what your attack stat will be if using point buy/standard array). AC on the CR table only increases (by one, might i add) at levels 4 (ASI 1), 5 (prof bonus to +3), 8 (ASI 2), 10 (a level after prof bonus to +4 - idk why it doesn't increase at 9), 13 (prof bonus to +5), and 17 (prof bonus to +6). AC on the CR table never increases after this point.
this shows us two things - one, that multiclassing and magic items were absolutely not things WOTC accounted for when building the CR rules (but we already knew that), and two, that you are, by the game's fundamental encounter math, absolutely expected to get your attack stat to a 20 by level 8. this is what i say when you "need" your attack/casting stat to 20 by level 8 - sure, technically you can get away with not doing it, but you are literally behind the game's math (however flawed you think that math may be) if you do so.
(this is also, again, why i find it absolutely ridiculous that WOTC refuses to separate ASIs and feats for One D&D - they literally designed the game around having ASIs, yet are still forcing us to choose between making our characters unique and following the game's most basic encounter math - the math that THEY MADE. it's absolutely absurd.)