Take note that ViaBTC is a mining pool and if you use their free accelerator and meet all the requirements, you still have to wait until ViaBTC successfully mines a block, and it can take sometime. ViaBTC also accelerates only 100 tx's per hour, so it is likely that you will fall under a queue.
I had a look into the blocks confirmed as at the time of this post it was at 815740 and the last block to be mined by viabtc pool is block 815721 which around 20 blocks down, imagine having to wait this long and if you don’t get into the next block mined by them you wait way longer again, definitely a long time before wait.
Although according to their FAQ you could get your transaction mined by other pools because they have a cooperation with other multiple pools but I doubt it will be for free services, it will certainly be for those paid services because they could be paying commission to those pools too. In order to pay miners it will be logical to just use the fee rate estimate by mempool.space as you said or wait till they get less congested a bit if one is not in a hurry to receive the transaction