Looking to open first stocks and shares isa and invest in global index fund..

After several years, having moved house again and having a steady income I'm hoping to finally start investing instead of dithering with cash sitting in an ISA. I'd like to put £10-20k in a stocks and shares ISA and a monthly amount, say £300. The plan is to put money away for university costs for the kids and later on to possibly help them with first home deposit, that type of stuff. I had come close to getting started at a couple of stages in the past, but the recent market drop has given me the impetus again.

After researching and spending time on this sub I'm planning to open one probably with Vanguard and put it in FTSE Global All Cap Index as there's less home bias than the Lifestrategy 100. As I understand it, if I'm going to invest passively and not buy individual stocks, there's little point in using another platform other than Vanguard for my ISA (eg. HL) as the fees will generally be higher?

Since nobody know when the bottom of this crash will occur, I was going to spread out the 'lump' payment. I was thinking like put £500-1000 every week. Does this sound sensible? One problem is I'm struggling to find out on the Vanguard website how flexible the payments are - it says you can open an account "for as little as £100 per month or a lump sum of £500". But can you vary the payments up and down during the year?

I feel nervous about leaving less than £10k in my cash ISA, but I know that it's losing money compared with inflation. Could I use my stocks and shares ISA to buy bonds with some of this (having more confidence since lower volatility) and sell some off if I did want to make a bigger expense, like buy a new car? I can't watch my ISA fester for much longer knowing that it's losing money, but need to have at least some cash available when needs must. I'm wondering what others would do.

I have always seen a new expense coming on the horizon and that's stopped me from investing. However I'm worried that the kids will get older and I will have left it too late if I don't act now.