I'm playing a Tome Warlock at the moment too, and I hear you. In combat I get, maybe, one spell, and then it's Eldritch Blast. The good news is that EB+Agonizing Blast is really good. My real pet peeve, as a GOO-lock, is that my spell list is overly reliant on Wisdom saving throws. It makes going against clerics and wizards tough.

Personally I always thought it was dumb and unnecessarily restrictive that your Mystic Arcanum spell slots can only be used for your one MA spell of that level. My house rules treat them like ordinary spell slots that regenerate after a long rest. So if you want to use your 6th level MA slot to cast Fireball, have at it. I don't make Warlocks (regardless of Pact) pay an Invocation tax for something that literally every other caster can do for free.

Also, rituals from every class are great. If you're DM isn't letting you learn new rituals on the regular, he's being a jerk. That's the main thing you do. My tomelock contributes quite a bit to the group with tiny hut, water walking and breathing, phantom steeds, detect magic, identify, comprehend languages, etc. etc. Gotta collect them all.

The real flexibility limitation of Warlocks isn't your spell slots. Multiple highest-level slots per SR is amazing. The problem is your spells known. I've introduced one more house rule, which is that Warlocks can "borrow" a number of spells equal to their Charisma modifier from their patron after completing a long rest. You spend 10 minutes per spell flattering your patron and telling him how great he is, and in return he lends you some spells for the day. These are in addition to your spells known.

The real question is, what spell lists can they borrow spells from? Obviously the Warlock list and your Expanded Spells are fair game, but what about other classes? If you want to make Tomelock's especially flexible, maybe let them borrow from the Wizard spell list. Or switch it up by Patron: Celestial: Cleric, Fiend: Sorcerer, Archfey: Druid, GOO: Wizard, Hexblade: Paladin (Smites!).

ncG1vNJzZmivp6x7prrWqKmlnF6kv6h706GpnpmUqHyurcqeZK2nnZp6uK3Rpaaco6NiurC%2BxGatnqqjnsGiuIycmKyslafAb4KScGlval8%3D