Advice for strategic investment ISA opening and future S&S learning

maxed my employer's pension contributions

Are you making a high enough employee contribution to avoid higher rate tax on your salary, allowances, overtime, taxable benefits, bonus, etc? Does the total level of pension (and future S&S LISA) contribution with modest growth above inflation and fees support your planned retirement date and income profile? If not it may be worth contributing more into the pension?

open a Lifetime S&S ISA to continue saving for when I'm 60 - probably with Nutmeg for the ethical investing option

Reassess the options when you are ready but Nutmeg are currently a loss making business and there is no cap on their fees (unlike HL and AJ Bell if you later switch from funds to ETFs) so would get expensive after a few years of contributions.

I need to open this ISA during this tax year if given that my plans are to open a Lifetime S&S ISA next tax year

I don't understand this comment - you are fine to open and contribute to both a S&S ISA and a S&S LISA in the same tax year (as they are different types of ISA) up to the £4k LISA and overall £20k ISA limits.

I have read about IFISAs and SIPP as well, but I don't know much about these.

These accounts are not necessary for now especially if your employer offers a good pension scheme.

Please can anyone give a view about whether Cavendish or Vanguard are good platforms for my £100+ monthly investment into a S&S ISA?

If you want to do this at high risk (eg 100% equities) it doesn't really make much difference. Vanguard have lower 0.15% platform fee but their global trackers are around 0.24% and Cavendish have a higher 0.25% platform fee but give you access to global tracker funds as low as 0.12%. I suggest you consider what the money in this S&S ISA will be used for as that might influence your desired volatility and fund choice. Cavendish would give access to various ethically filtered options.

Alex

At this level a financial advisor is unlikely to be interested and even if they are will likely be uneconomic for you. I'd agree with Alex that contributing more to your pension is worth reassessing if it is a good one. LISAs can be accessed at 60 and by the time you get there pensions won't be much different. Your company pension is very likely to have caps on charges and can often negotiate discounts. Can you contribute via salary sacrifice? I would avoid IFISAs completely, they are not suitable for retirement planning

You can never learn too much about investing and even if you don't construct a bespoke portfolio it is reassuring to know what's what. Other than that it seems a sensible enough plan for now