A small desktop fan blowing in to the system is your friend.
I needed this many years ago to keep a system's CPU below 80°c when working hard.
:-)

I try to keep my temperature at around 60 °C. When I limit cores to one I can indeed reach 100% without going too far above 70 °C.
But I can also run BOINC on both cores and set usage safely to 70%. This is better, isn't it?

Not really.
Making use of "Use at most x% of CPU time" to reduce the load on a CPU actually puts more stress on the CPU as the constant starting & stopping actually puts quite a bit of thermal stress on it- it gets hot, then cool, then hot, then cool, then hot then cool. Expand, contract, expand, contact, expand, contract...

Why? Wouldn't give me a 1d WU give me a farther away deadline?

Nope.
The deadline is fixed, that is the period of time in which to return a Task. If you have it set to run for 24 hours, and then make use of "Use at most x% of CPU time" to keep your CPU cool, as you have found that increases the time it takes to finish the Task.
Hence why going with the default Target CPU runtime, making use of just the 1 core & setting "Use at most x% of CPU time" to 100% would be your best option- keep the temperatures down, get plenty of work done, and not run in to deadline problems.

What temperature is the CPU running at? As long it's 70°c or lower, it's not an issue. From memory, even with the stock heatsink & fan, even running Rosetta 24/7 shouldn't put it's temperature over 70°c as long as the heatsink & fan is clean, along with the inlets & outlets of your case and the case fan(s).

The other option would be let the Tasks run uninterrupted, but only use 1 Core of your CPU. More processing would get done, and you'd still keep the CPU cool.

Use at most  50 % of the CPUs
Use at most 100 % of CPU time

Well, the task is running and it's checkpointing fine.
Is it right, what Mod.Sense worked out, that you've changed your preferred runtime to 24hrs rather than the default 8hrs?
Because if you've also set it to suspend running while in use, that's going to extend the runtime to exactly what you're seeing and sometimes you'll struggle to meet deadline.

The task has run successfully for 17hrs. Set a more appropriate preferred runtime (and the default 8hr suits you) and it should report in a time acceptable to you, before deadline.

You asked at the start if it's the taskproject or you. It's your amended settings.

CPU time			07:12:10
CPU time since checkpoint 	07:12:10

Well, it's definitely running, but it's getting interrupted quite a lot (24hr difference between the two)
Do you have "Suspend when computer is in use" checked?
What's the time since the last checkpoint? Has it checkpointed at all?
I'm guessing this must be one of those 16hr (cpu time) tasks otherwise the watchdog would have cut in already
It looks like another dodgy task - it's not looking good

Don't worry about old tasks too much. They do seem to be aging them off quite quickly, I agree

Application 			Rosetta 4.07 
Name 				3az4ii6b_jhr_design1_COVID-19_SAVE_ALL_OUT_903430_1
State 				Running
Received 			Sun 29 Mar 2020 05:38:35 CEST
Report deadline 		Mon 06 Apr 2020 05:38:34 CEST
Estimated computation size 	80,000 GFLOPs
CPU time 			17:05:56
CPU time since checkpoint 	00:04:11
Elapsed time 			1d 19:53:47
Estimated time remaining 	19:00:07
Fraction done 			69.789%
Virtual memory size 		1.32 GB
Working set size 		1019.19 MB
Directory 			slots/1
Process ID 			13540
Progress rate 			1.440% per hour
Executable 			rosetta_4.07_i686-pc-linux-gnu

Weird. If you click on the task and select properties, what does it show for CPU time and Elapsed time?
I'm wondering if it's running at all?

CPU time 15:29:54
Elapsed time 1d 15:25:10
Estimated time remaining 22:53:40
Fraction done 63.260%
Progress rate 1.440% per hour

Please, read other thread
Rosetta Mini for Android.