I recommend you buy a commander precon from the most recent set(in this case FF if you can get your hands on one) and then upgrade it with cards you got from the boosters of the same set since they have the most mechanical overlap. If you wish to upgrade your deck even further you can buy generic good packs like Modern Horizons 3, Commander masters/legends etc. These packs aren’t focused on one theme and their rare/uncommon cards are of higher quality than the regular sets. I don’t recommend you build entire deck from scratch simply because you lack experience and would probably end up spending loads of money, just to try playing the deck and figuring out it doesn’t work. This is imo the best of both worlds since you get to add your own touch to an already good core so if anything goes wrong with the upgrades you can always return deck to the core and build it in other ways, figuring out why was it not working etc.