Mordred vs. Lancelot vs. Rama

So, the easy answer is it depends on what you have. I think leveling Mordred for farming purposes is safe, and either Rama or Lancelot (Both are crit focused, Rama is buster Lancelot is Arts,) depending on what your team comp looks like. Of those two, Saberlot is better, i think, but if you run a lot of buster support Rama may be a better fit.
That said, if you are going to level Gawain, Mordred may not be a priority either- she's better, objectively, but always go with your favorite.

I’m in a somewhat similar situation to you, as I recently rolled both Lancelot and Altera. I recommend prioritizing Mordred first; even if she doesn’t have as defined of a role as Lancelot or Rama, her base stats are just too good not to level her.

I think Lancelot vs. Rama depends a lot on the supporting CEs and servants that you have. In general, Lancelot is more self-sufficient (his kit gives him NP charge, stars, star absorption, and critical damage up) whereas Rama can produce only star absorption and critical damage. Rama is also built for high damage bursts; Lancelot can give better sustained damage over time. If you have the right Arts team, Lancelot can be lethal. I only have him at level 60, so I don’t have his third (and best) skill yet, and he can be a monster. I run him with Leisure Stroll CE to boost Arts performance and ensure he gets stars when his skill is down, Emiya Assassin with Be Elegant, and Hans with Fragment of 2030, which pretty much ensures that Lancelot always has a high chance to crit. In one case, I pulled off an NP+A+A brave chain that killed the first enemy, Lancelot turned to the second enemy, crit and overkilled on both of his Arts cards, and was back at 100% NP at the end of the turn. It was disgusting.

So, if you can build an Arts team around him, definitely choose Lancelot over Rama for priority number two. I also think his role (crit Arts, ST NP with decent spammability) is different enough from Mordred that it’s worthwhile to level both.

If I had to pick one I'd say Lancelot. Having a farming servant is nice and I recommend eventually raising Mordred but Lancelot is a power house that rivals some ssr's even and that might be helpful in the coming events maybe. Also from what I was told buster crit, while still possible to do, does not really take off until Merlin releases. Whereas arts crit I think works wonderfully with just Hans and Mozart who are available now

I'm in a similar situation. After completing Camelot with no sabers at all (I legit started levelling bedi after clearing Camelot) I got umu saber and now recently dadcelot saber.
While I levelled umu first because I only got dadcelot recently and also because well, umu, I plan on doing Lancelot next even over completing my cu and levelling my two 5* riders iskandbro and oppai pirate. Dadcelot is just that good at his niche roll and his ST NP smacks. The sheer ability to gain and use stars to crit only adds to his boss killing ability and when coupled with an arts card or crit damage buffer he can shred bosses.
The CE gamepress suggested for him is probs the best one to use if you have it. Another ending does feel like it was made for him.
Moedred on the other hand is worth levelling imo for waifu purposes (her fourth ascension card art is really good!) and for the chichiue memes. Plus as an SSR she has naturally good stats and a hard hitting buster.
I also agree with the consensus that Rama probably needs Merlin out to get full use of his buster crits. Shakespeare does ok but only ok.