Pros and cons to both, and it all depends on what your priorities are with your new setup.
Just a heads up before deciding to or not to buy Valencia. Valencia is $2k while still requiring a preamp and DAC. It has also been mentioned in another forum that its sound is highly influenced by the rest of your chain. So it might be another rabbit hole for you.
Mjolnir 3 I find enjoyable, but it is quite a hefty headphone amp and puts out a lot of heat. I currently have mine paired with a Yggdrasil+ MIB (some others are using variants of the Yggdrasil or G2). Not necessarily needed, but Mjolnir 3 pairs well with those DACs.
There have been quite a few AIO solutions rolling out lately from various manufacturers in both Tube hybrid and SS varieties. Some of them have swappable DAC cards.
Lots to consider prior to making your decision, but take your time weighing the options. It will save you from regrets later on.
Good Luck, and Happy Listening!
Well the simplest I could go would be a dongle DAC/amp. I have a few now and I would have no problem with simply using one of those. My main issue I seem to have right now is my Ares II. If I have a tube and SS amp connect I only get sound from the SS amp if the tube amp is turned on. I don't seem to have this issue if two SS amps are connected to the Ares II. I want to have at least one tube and one SS amp in my setup if possible.
I mention the Valencia as so far it has very good reviews. Zeos being one of them but I believe he had a hand in the design of the Valencia.
My SS amps currently are the Rebel Amp and Musical Fidelity MX-HPA. Both are great. The "downside" of the Rebel is that it's only SE in and out. Not even XLR for convenience. I've looked at various brands like Schiit, Fiio, JDS Labs and Geshelli. Both for DACs and amps. Definitely lots to consider and choose from.