question

Literally no problem with this; the ease of setup will depend on which lithiums you use, specifically if you're using drop-ins or component cells that require a separate BMS.

If you're using drop-ins like Battle Born (Firefly) or Victron's drop-ins or any number of others, it's literally only a matter of changing the charge profiles of your MPPT and your Multi to match the manufacturer specs of your lithiums (though the default lead-acid profile in the Multi is frequently a great match for lithiums, it just depends on the brand and etc.)

If you're using component cells and a separate BMS, there's an added degree of complexity in programming because you just have to make sure that all the components are talking to each other, and the degree of complexity depends on the battery specs and the brand of BMS and etc.

Ultimately, though, the answer to this is simply a resounding yes. This is not only not a pipe dream, this is actually very common practice. Depending on where you are, you can give us a call here in San Diego for Battle Born pricing, but you may well be able to find them somewhere closer to your current location, and in reality there are several other brands of drop-in lithium battery that are excellent as well; of course, this being a Victron forum I would be remiss if I didn't mention that we can get you Victron drop-ins or component lithium systems too, though the specs are different and don't meet everyone's needs.