Best paid gacha for me?

Seriously you have 4 5* servants within 100 days? I only have two -,-

I am f2p afterall, but definitely I will buy this summer GSSR. Actually, I am in the same position as you. My knights are terrible, plus I don't have any extra other than Mash. While the horsemen (cavalry) I have Ozy and Raikou, and I have several good 4* to fill Caster. Tbh, I only need Jack or Waver or Tamamo. Most people will say "choose horsemen", and since I do want horsemen, I will definitely choose it. I just worried if I got another Berserker/Rider, because right now I don't think I need additional Berserker and Riders. My raikou and beowulf pretty much deal with all of common enemies.

If you truly need knights, save up for Camelot 2 banner or Class Based Summoning Campaign. Beside, yours is enough. Mine? Hahaha, my only 4* on Knights is d'eon. But yeah, he save my ass hundreds of times because I build my team around my Berserkers

Take this with a grain of salt since I think I'm pretty biased when it comes to this, but if I were you I would still roll cavalry. My reasoning for this is that the cavalry banner contains some of the best servants in their respective classes so far, whereas the knights banner's servants are somewhat lacklustre in comparison. Also, your knight roster is fine as is. Yes, your cavalry roster is (much) better, but you have plenty of time and banners to get better knight class servants in the future (ie Gilgamesh, Tamamo lancer, Okita, Saberlot and the like who are all better than most of what the knights banner has to offer).

Also, a 1/10 chance at Cu Alter, Waver, Tamamo, Ozy and Jack is imo too good to pass up, especially since you don't have any of them. That said, you've already got Xuanzang and Drake, and as far as berserkers go, Nightingale and Vlad aren't amazing (but still better than Beowulf and Fran imo), and Medb is straight up bad, so there's a also a 1/2 chance you will get something you may not want. That's the risk you'll need to balance.

Lastly, there is nothing in the game that requires you to have better servants in the knights classes than what you currently have. Rama is a solid 4* saber, and Altria is a 5* so you're set there. Cu is still considered one of the best lancers in the game, even if it's just because there aren't many good lancers in general, and Emiya becomes a good crit archer when he gets strengthened later on. Anything else you might be missing can easily be found on the support list.

Basically, are the 5 best servants in the cavalry banner (plus Vlad and Nightingale if you want them) worth the risk of getting duplicates and passing up the knights + Jeanne?

Your Cavalry roster seems pretty good already imo.
But I'd still recommend rolling on Cavalry for the sole existence of Waver. Not to mention most of the other available units are the best of their class.
Ozy, Waver, Tamamo, Jack, Vlad & Cu Alter are all excellent units that are rated high in tier lists for a reason.

Compared to that, most units in the Knights banner are meh. The only exception to that being Orion and Jeanne.

To be honest with you, Cav has a much better roster, but you seem to have a pretty good selection of servants in general, so whichever has banner has more servants you'd appreciate is probably the one you should roll on.

For me personally, I'm stuck between rolling Knights for a chance at Mordred or Jeanne, or rolling Cav for Sanzang or Ozy

Your knight is lacking in comparison, but the gacha does not have the best knights. The best ones are limited. That said there is no guarantee of pulling said limited knights so pulling for knights now is an option