Nici,
If you are looking for a faster paced grammar, then PLL and English for the Thoughtful Child are both going to be a step down from Rod and Staff in rigor. Both are more CM in their focus with very little in the way of formal grammar instruction.

While we love much of CM's philosophy is so many areas, this is one area where we differ. At HOD, we do delay formal grammar instruction just slightly in the early years, starting off gently, but in the long run we want a deep, solid, thorough language arts experience, including all of the instruction that falls under the "English" umbrella. So that is what we schedule throughout our guides.
FLL is mainly a grammar program, so comparing it to Rod and Staff is like comparing apples to oranges. You can see my thoughts on this in the post linked above. I used FLL 1 and 2 with my oldest son and have now used Rod and Staff with him for levels 4, 5, 6, and into 7. Rod and Staff is an English program, incorporating writing lessons, grammar lessons, dictionary skills, prefix/suffix work, poetry, etc. and much, much more. It is truly a full English program. So, if you're looking to use a replacement for Rod and Staff, you'll need to replace all of those areas.
In looking at CQLA, you will find that it is a very time intensive program in many areas. It has significant overlap with much of what we are already teaching and scheduling within our guides in many areas so that is something to weigh as well. For the balance that we are looking for in our guide, we find Rod and Staff to be the perfect fit and complement from year to year.
While grammar is one of the easier areas to supplement or switch out without affecting the flow of your HOD day, it is also one of the most important areas and one that we ponder deeply as we plan each guide. It is possible that by using a different language arts program, you may miss some of the areas we are planning on having covered through Rod and Staff or through the other language arts areas scheduled within our guides.
From what you've shared, it sounds like you may be at the beginning of Bigger Hearts and possibly at the beginning of English 3? If so, it does get more challenging as the year goes on. If, instead you have been doing English 3 for awhile and you are in need of more rigor, our best recommendation would be to move up one level of Rod and Staff to level 4. Make sure to do all of each lesson. There is quite a jump between 3 and 4.
I can understand the desire to have grammar be fun and exciting, as I was on that search myself with my oldest son for years.

We did FLL 1&2, then a year of Easy Grammar while using Intermediate Language Lessons for writing, then a year of BJU English and Writing, and finished with part of a year of a "fun" program with cute books and workbooks to learn the grammar concepts. I finally discovered after all that search for fun that my son had retained very little grammar knowledge and no application. He did not see the need for grammar instruction or the reason it was necessary.
At that point I switched to Rod and Staff 4 with him when he was a 5th grader. I cannot tell you the difference in understanding within just a few days, literally.

The way Rod and Staff presents their lessons truly speaks to the reason for knowing grammar and all things related to the study of English. It points out that since God himself chose to speak to us through the written word, it is very important for us to be able to convey ourselves in this manner well too. All throughout God's word, we find that specific wording, or the way things are phrased, or the word order makes a huge difference in our interpretation. This is the reason for studying diagramming, as it teaches all of those skills. It helps students visually see a sentence and its design, ponder the specific word choices, see the way things are grouped and phrased, and grasp the meaning.

I have now switched my goals from looking for a fun program that entertains the children to one that provides purposeful, solid, specific, God-honoring instruction in English and writing that speaks to the heart of my children. A side benefit is that it also prepares my children well in so many areas regularly tested each year. We have been blessed to have found that within Rod and Staff.
While you can use a different program to fulfill this area if desired, the HOD Board is focused on helping you use the language arts components scheduled within the HOD guides. Should you desire to pursue a further discussion of a different program than those we schedule, you'll want to take that discussion to that publisher's board or website instead.
Blessings,
Carrie