Would taking AS Level mean taking fewer papers in A Level next year?

Depends which A levels you take.

If you take the same A levels used in schools and colleges in England then No. The AS is a separate qualification that is optional and you would be tested on all of the 2 years of content in the full A level exams.

A levels in Wales would permit the AS / A2 split.

If you opted to take the International A level exams from Cambridge or Edexcel then you can split the AS exams from the A2 exams to spread the workload. Edexcel are totally modular with more then one exam opportunity each year. You'd have less choice on exam centres and the prices would probably be a little higher but it might better suit your needs. Tutors and Exams are the only centre I know of who are offering the Edexcel modular international A levels.
https://www.tutorsandexams.uk/

Thanks a lot for your help!

I haven't decided yet but I'm residing in England, so taking Wales A levels won't be an option. So it would be a choice between A levels(England) vs International A levels.

I've been looking at online courses like these. These are A levels taken at schools in England, right? (Although CS one's exam board CAIE's 'I' stands for 'international'.

Also, is it actually possible to take International A levels as a private candidate but also as a resident of England?

Edexcel website says "Pearson Edexcel International AS/A Level qualifications are available to students attending International Schools (outside the UK) and any students attending Online Schools. They are not available to schools in the UK, the Channel Islands, the Isle of Man or in British Armed Forces schools overseas." https://qualifications.pearson.com/en/qualifications/edexcel-international-advanced-levels/about.html#:~:text=Pearson%20Edexcel%20International%20AS%2FA%20Level%20qualifications%20are%20available%20to,British%20Armed%20Forces%20schools%20overseas.

Tutors and Exams website also says "Tutors & Exams are able to offer Edexcel International A level exams in the U.K for those students who:
Are studying International A levels in the U.K through an International Learning Provider / Online School.
Have been studying Edexcel International A level subjects internationally but wish to take the exams in the U.K."
https://www.tutorsandexams.uk/exam-centres/what-exams-can-I-sit/#ia-levels

Sounds like International A level exams are not an option for me.

Computer Science - CAIE - is an International A level that can be taken in some UK exam centres. OOL have in the past helped candidates locate potential exam centres.

NEC also offer Maths and Further Maths (Edexcel) https://www.nec.ac.uk/courses/maths-a-level/ Although Maths and Further maths are completely separate qualifications you might want to choose to stick with the same examiner for both.

Talk to Computers and Exams. Edexcel International A levels were available through them to independent candidates as long as they were not in mainstream education. The latest wording on their web site has changed and so the restrictions placed on them might have changed.

Huh, anything that CAIE provides is International A level? They don't seem to provide just 'normal' A level -- their website only shows International A level. That's unexpected. https://www.cambridgeinternational.org/programmes-and-qualifications/cambridge-advanced/

Thanks for the advice as well. May I ask why is it important to take Maths and Further Maths from the same exam board? Is it because of the exam styles? Or is it scope?

CAIE only do international A levels.

Every examiner has their own style. They also have a set of text books that present content in a consistent way. It is up to you if you want to mix and match Maths and FM examiners but sticking with one, especially as an independent candidate, is likely to be beneficial.