Ab-initio Russian

To be honest, Russian is a difficult language and it's much harder than French or Spanish. It has 3 genders, 6 cases, difficult pronunciation, vocab that mostly doesn't resemble English in the slightest and its grammar can be pretty alien to English just in general. Of course difficulty is subjective but Russian is generally seen as a hard language for a Brit to learn. Personally I do find Russian quite difficult and it's much harder than French/Spanish/Portuguese where I could generally learn the vocab and form a relatively coherent sentence by just following English grammatical structure.

However, you shouldn't let this put you off it if it's what you want to achieve. As hard as Russian is, obviously it can be learned and there are aspects in the language that are arguably easier than in English/French/Spanish such significantly less tenses (although imperfective and perfective verbs can be quite a strange and difficult concept to get your head around). Plus, you'll have 4 years at university including a year abroad where you'll intensively work on your Russian which will obviously get you to a high standard in the language.

As for general advice where to start, without stating the obvious like learning basic phrases such as здравствуйте, спасибо, пожалуйста etc. or how to count to 10, some concepts I would recommend familiarising yourself with are:

Genders (what makes a word masculine, feminine or neutral)
Conjugations
Start learning the uses of the 6 cases (you don't have to delve into all the declensions immediately but familiarising yourself with what each case serves for is a good start)
Past tense
Understanding the difference between perfective and imperfective verbs as well as omnidirectional and multidirectional verbs
Learning the basic phonology such as tonic vowels (e.g. an unstressed "o" is pronounced as an "a") and what soft signs are

To help you in the long run, I would strongly recommend that you learn individual words with their respective stress e.g. learn that it is здрáвствуйте rather than здравствуйте

Some resources I'd recommend include:
https://www.russianlessons.net/
50languages.com
Memrise
Duolingo (unoriginal but it's good for learning how to structure sentences. But make sure you use the web version that includes "notes" which explain the grammar better although I personally don't think Duolingo is great at teaching Russian grammar)

You sound experienced in language learning so you probably already know that it is best to use a variety of sources so don't make the mistake of using Duolingo and nothing else xd

I'm aware that I've written a lot and have probably waffled on as well as maybe having missed out some important points but these are the most important points that came to my head. I hope to have been of some help and удачи в учебе!))

Ah thanks so much for all the advice!

I'll take a look at those resources and go from there. Hopefully I don't do too badly.

(And yeah honestly I dislike Duolingo haha, it doesn't get you too far on its own!)