The following is only if you want to keep in contact with your father and his wife. It will be difficult to do initially but by giving them and yourself boundaries your time together will eventually become more pleasant and more respectful.
You are now the adult in this relationship with your father and his wife. You have to act like the adult and stop acting like the child.
Adults have schedules because adults lead busy lives
Make a schedule that you feel is reasonable for your time and then cut that down because they will eat your time back up. One two hour block per week, one two hour block per two weeks, whatever works for you and you are comfortable with doing that is a small infringement on your life. Divide your comfortable time in half and give them that much. It will eventually go up to the original number, but you need to have that ceiling firmly in place.
Adults keep their schedule because adults understand their word, especially their word to themselves is what is truly valuable
This is the hard part because you will now be playing chicken with them, first one to swerve loses. You keep saying no until they swerve. You will know they are swerving because they will be honoring the schedule you have laid out for them with you. Just say no until they are asking you to do things within your parameters. Then you keep your parameters tight. Do not blur the edges, especially at the beginning, keep it clean and on-time, no extra stops, no picking up groceries or medicine if it's not on the schedule for that day.
Keep it within the timeframe. You are driving. You literally control all of it. If a medical person comes out to tell you to pick up something for them or stop somewhere, instruct them to put it into writing so that their patientyou are neither the doctor not the caregivercan make arrangements to get what is needed. This is you adhering to your word to yourself. You made the schedule, you keep it for you. People will ask what's your schedule, where do you have to be, none of their business. You did not ask them for help in scheduling your time, but thanks for the offer. Do not give explanations. You don't owe anyone an explanation for how you spend your time. If someone asks, shut them down, tell them it's an inappropriate question.
Adults have to be the ones to uphold the rules
They will beg and plead for you to increase it, the answer is no, when they ask why, tell them because you said so, and to stop asking. If you wanted to share with them you would, otherwise, it's none of their business.
They will ask you to change it three to ten times per week. You keep saying no to changes, please stop asking, and, eventually, they will stop asking.
Once your father and his wife realize this is your schedule, and these are the rules, they will schedule their appointments around your time. Even if something urgent comes up or that's the only time the doctor has open, whatever it is, they can take a Lyft or a cab. They can count their own money which means it's not your problem, unless those types of tasks are becoming increasingly difficult for them then that's a whole different animal requiring a diagnosis and other measures.
Adults use time management for better living
Limit call time to you to once per week, for a certain number of minutes on a certain day and at a certain time. If it's five minutes you hang up at five minutes. It helps to give them a countdown, It's one minute until I hang up, it's 30 seconds, 10 seconds, leaving now, good-bye. Don't answer calls from them after that.
Prior to engaging, let them know they are contacting you too much and it is non-productive. Tell them the new system is they write down their questions, information, etc., and tell you during your weekly call. Explain their constant communication is invasive to your communications systems, and unproductive for you and them since you don't read or listen to these messages anyway (even if you do read them) it's fills space, so you will block them all week until they cease this behavior and keep their outreach to you on the one day per week you communicate with them.
Block calls and texts until next call-time, but before you do that, remind them your next call time will be on this day at this time for x number of minutes and until then you will not be available, by phone or text.
Once you have them on a schedule, if and only if you want to do this, you can increase your time driving them to appointments. This is not a reward, it is about you and your relationship with your father, so do it only if you want a little bit more time with him.
Adults recognize and understand when they are the adult in the room
Think of them as toddlers, you have to say no one more time than they ask. Toddlers don't reason well. They want what they want when they want it. You have to change their behavior by changing your own.
In other words, given their stage of life and how they are beginning to move through it, you now have to become the adult in order to keep this relationship.