Investing at 18

so basically im 18, just got my first job and im trying to get into the habit of maxing my ISA every year for the next 20ish years

the issue is theres only about 8 months left in the tax year so if i want to hit £20k i'd need to put in around £2,500 a month which is basically everything i've got. like it's doable but i'd be eating rice and prayers for a few months lol

the other option is i just put in what i'd normally put in if i was doing it over 12 months, so like £1,667/month. way more comfortable but i'd only end up putting in around £13k and lose out on about £6-7k of allowance this year

once i'm on a proper 12 month cycle next tax year it's fine, it's just this first year where i'd have to go a bit mad to hit it

my plan is to max it every year going forward so in the grand scheme of 20 years does that ~£7k from year one even matter that much? or am i overthinking it and should just contribute what's comfortable and crack on

this is all my savings btw so yeah bit scary either way

also, anyone here just go 100% VWRP and call it a day? i keep reading that at my age i should just throw everything into a global all-cap accumulation fund and not think about it for 20 years. is it really that simple or am i missing something going all in on one fund?

any advice appreciated cheers!