There was a recent thread about liners recently that might be useful. Erm…
https://singletrackworld.com/forum/topic/recommended-liner-undershorts-please/
Personally I’d just get some decent lycra shorts and use them as a liner – I like the DHB Aeron ones. The Morvelo ones are kind of okay, but the trade-off with liner shorts in general seems to be that the pad simply isn’t as comfortable as those in proper shorts. The DHB Aeron pad is really comfortable ime and far better for longer rides than any liners I’ve used, though more expensive. More durable too, thicker fabric gives better support and wear resistance.
A lot of people will say ‘bib-shorts’, but for mountain biking I’ve found waist shorts just as good, plus more breathable for the torso in hot conditions and more convenient for calls of nature.