firstly, congrats on your gcses, you did so great!!! i just finished year 12 taking maths, fm, physics and chem, so i can advise a bit on maths/fm and taking 4 subjects in general.
i agree completely that three good grades are better than stretching yourself too thin, but if you would like to take further maths, i think it would be a good idea to start out with it and see how you cope - you always can drop it. doing further maths definitely cut down on the amount of work i had to do for normal maths by quite a lot, so that could be a consideration for you. maybe you could think about doing AS further maths and dropping it if it's too much? i'm not sure how beneficial this would be for architecture, but if you really wanted to take fm then this gives you a qualification at the end of the year. i found it quite tricky at first to balance 4 subjects and volunteering/supercurriculars, but it's absolutely possible if you manage your time well. i tried to stick to always having all my notes/anki for the week done on/by the weekend, so that work didn't pile up and i could just focus on doing past papers when exams came around. i certainly didn't use every single free i had, but it's good practice to try to utilise that time wisely, to cut down on the amount of work you have to do at home
for maths and further maths, it's always good to complete some questions from the mixed exercise at the end of each chapter: there'll be some cases in which you understand it easily and can just choose a few of the harder questions, and probably other times where you might want to go on PMT to do some more practice questions. as long as you try to make sure you can do questions for each chapter as you go along, you should be fine - don't let it build up! there were some chapters for further maths which i really did find quite difficult, but if you have helpful teachers - or even your classmates - don't be scared to ask if you don't understand something. a good thing about further maths is that (for edexcel), the structure of the papers tends to be fairly consistent. so, while there will always be one or two questions that throw a lot of people off, if you learn the fundamental skills, most of the exam papers should be accessible
i hope this helped, and best of luck for year 12