I've been a nanny for 15 years, and this is a routine that's worked well for me:
3:15 - school pickup, snack, then straight to the playground that's nearest school, in all but the most extreme of weather. Get waterproof suits for all of you. The fresh air and running around is vital for releasing the pent up frustrations of the day. Run around and join in with the play as much as you can - you'll get endorphins too that way and feel happier and calmer later in the evening, and it's a good way to show your eldest that they're getting your time and attention too. Usually, especially in summer, there will be other mums at the playground who you know - chances are there'll be one who's happy to make a fuss of your littlest while you take the opportunity to run around being a pirate or a superhero or whatever. If not lug him round in your arms and make him a character in the game too (subtly suggesting to your eldest that he's fun to have around!)
After 30 - 40 minutes, head home. If it's a drive back have an audiobook or nursery rhymes on in the car so your eldest can turn his brain off a bit.
As soon as you're home, get the reading out of the way. If you can put your youngest in the playpen with some 'high-value' things to explore. By 'high-value' I mean coveted, not your usual baby toys - random things they wouldn't normally get to play with. Suggestions: Spice jars and lids (don't wash them out , just brush out any remaining herbs or spices, so they still smell strong), egg box with realistic rubber eggs and a couple of wooden egg cups, and old handbag containing 'keys' (look up yummi keys on amazon, add a leather keyfob, maybe a couple more safe keyrings) and a wallet with expired credit cards and monopoly money, and an old dead mobile phone (they love those little flip ones) plus a couple of other random bits, a gallon bottle filled with water (maybe superglue the lid on!) a busy board (or handheld activity cube would be the budget solution!) a mesh bag they can open with a drawstring that contains lots of different kinds of fabric offcuts (too big to swallow!), a seagrass basket with a bunch of either naturally occurring or made from natural materials stuff, that sort of thing. Or high-value toys. But the idea is he ONLY has access to these things in the 20 minutes you spend doing reading with your eldest, and only a couple of the things each time so they stay interesting. Hopefully then he'll be distracted while you're doing the reading.
After reading, little one goes straight on your back in a back carrier (I love my Manduca for this) while you crack on with tea. Ideally you'll have something in the slow cooker or you can defrost something you previously prepared, but if not try and keep weekday meals quick and easy - cheese on toast, jacket potatoes, garlicy flatbread and carton of fresh soup, pasta pesto (see username!!), vegetable fritters, fish fingers and peas, all that nice simple stuff, so little one doesn't get bored on your back. While you're doing this eldest can be at the kitchen table doing homework, and you're handy to help if needed. With your youngest on your back it'll feel more like you're concentrating just on him. A fun teether toy tethered to the carrier should help keep littlest on distracted! After homework if tea's not ready your eldest can chill out doing jigsaws or drawing with an audiobook on, or watch TV until tea time.
Serve up tea. If your eldest needs to practice sounds or sight words or anything turn it into a sort of game to do with them while they're eating (unless you eat with them of course). It's less of a chore and more enjoyable that way. You can involve your little one too, by showing them a flashcard with a picture on and asking them 'what's that?' on their turn. If they aren't verbal yet you can ask eldest to help them answer 'A Cow!' or 'A Jug!' or whatever.
Chuck them in the bath. You can make bath time a nice long activity rather than a quick dip if they generally get on ok in the tub together. It's nice for them to have that positive bonding time, as the aim of the afternoon up until now has largely been to keep them apart!