There are a few options you have here and 500€ is a good budget. You want good coffee throughout the day with the least amount of work possible. Also I assume you are looking for large cups of coffee since you tried instant and french press. So you should forget about espresso machines and forget about anything handmade. You need something automated for sure.
Option A: Espresso machine
If you want coffee drinks with milk in it the way to go is to get an espresso machine. However I think for you it's not a good solution for the following reasons.
- It's quite complex to get into and to be able to make good coffee. It doesn't go well with your time requirement.
- It is much more costly than the other options. Grinders for espresso start around 250€. It is in my opinion the minimum you should invest for good results, although more would be better. Decent espresso machines for home use start at 500€. It's also the minimum I would recommend you to invest. That's already way over budget. A decent alternative would be to get an electric burr grinder, a good stove top coffee maker and a milk steamer.
Option B: fully automated bean-to-cup machine
In my experience the coffee these machines make are pretty awful. Convenience is their strength though. A fully automated machine that makes milk based drinks automatically will set you back as much as the espresso machine/grinder setup. Though at a much lower drink quality. Jura is a famous brand for these machines. I think they may be a good solution for larger offices where everyone has different preferences as you can make black coffee as well as milk based drinks. I think you specifically have to consider the trade off between price, quality and convenience. If convenience is your main consideration you might be happy with one of those.
Option C: Capsule machine
A typical consumer grade, capsule or pad/pod based coffee machine is as simple as it gets. The time you need to prepare a cup of coffee is probably shorter than with any other system at the price they are charging you for the machines (you can get them for 100€ easy). However there are some downsides that disqualify these machines in my opinion.
- The price per coffee is way too high.
- They produce an insane amount of trash (this can be avoided with refillable capsules at cost of convenience).
- The quality of the coffee is pretty low.
I would argue this doesn't fit your criteria since you won't get high quality coffee (plus the other downsides). I would probably prefer it to a fully automated machine though and invest some of the saved money in a decent milk steamer if you want to make milk based drinks. It's slightly more work but much cheaper.
Option D: Filter coffee machine
I believe this is the way to go for you. It's within your budget, fits your criteria of low maintenance and good quality and it supplies you with coffee for the entire day. Get an electric home grinder and a good filter coffee machine. For 150€ you can buy a very good electric home grinder (check Baratza) and for 200-250€ an amazing filter coffee machine (Check the SCAA approved home drip machines, I recommend Moccamaster). Handling it is extremely easy. Fill the tank with mineral water, grind the coffee (weigh it for optimal results), put it in the filter basket with the paper filter inside, turn on the machine and wait for 2min. There are many advantages over any other system when it comes to your criteria.
- Preparation time is low. Preparing a whole pot of coffee will only take you a couple of minutes of which you only actually work for maybe 30s.
- Cleaning is extremely simple. Throw the paper filter with coffee grounds and flush the basket. Occasionally put the basket in the dish washer. Same for thermos. Flush with warm water and occasionally clean with soap.
- Preparation is extremely simple. You basically can't mess it up as long as you follow a recipe (coffee-water ratio) that the roaster can recommend you or that you find on the internet, because all the variables are preset by the machine.
- If you get a good filter coffee machine with a good thermos you will have warm coffee for half a day. So say you make one pot in the morning for breakfast. That will last you until midday. Prepare another one after lunch which will last you until the evening when you finish work.
- The coffee (provided that you buy good beans) will be far better than with any other automated solution.
With your budget and your requirements I think getting good milk based drinks is difficult. Quality will always suffer greatly. If you insist on it though, I think you should buy an electric grinder, a good stove top coffee maker and a milk steamer as suggested in Option A. A bit more time consuming but better coffee as the other options (except the espresso machine).
Hands down though a filter coffee machine is the way to go if you are willing to adjust yourself to black coffee. It's healthier anyways and I believe anyone who is willing to try drip coffee will come to realize that it's the far superior way to consume this wonderful beverage.